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I'm focusing on the question why the opinion of the church changed in the past, not the "historic landmarks" (single events) that lead there.

I'd like to look on two periods in time:

Before AD 313 (edict of Milan)

The situation then was completely different than the situation today:

• There were many contradictory doctrines (example: Arian controversy) and unlike today there was no authority deciding which opinion was the "official one" when there were contradictory opinions.
  So we cannot say which one was "the opinion of the church" when there were both voices favoring and rejecting capital punishment.
  We know that many Christians in the first centuries saw military service as sin. Maybe they also saw capital punishment as sin.
• People working in the legal system were (nearly only) non-Christians. So seeing capital punishment as "allowed" does not necessarily mean that Christians are allowed to be involved!
  However the catechism (today) can only answer the question if Christians are allowed to be involved.
• Other forms of punishment simply were not known or they were not feasible in that time (thanks Thunderforge for your comment)

After AD 385

According to Wikipedia no later than AD 380 the Roman government started to influence teaching of the church in major aspects. No later than AD 385 Christians having a different belief even were executed.

This means that Christian teaching was massively influenced by people we would call "politicians" today.

In later periods this was even worse:

In the medieval period for example bishops typically also were politicians. You might find this "polemic" but unfortunately they often adapted Christian teaching to their political interests and not the other way round. (There are for example paintings from that period showing this.)

Today

Today we have a completely different situation:

• The most important: Christian teaching is not influenced by political or other interests (at least I hope so)
• If the church would say: "Capital punishment is allowed" this definitely would mean: "Christians are allowed to be involved"
• There are other methods of punishment which work well
• Although there are many different opinions in the Catholic church (as a Catholic I know this!) there is an authority defining which of these opinions is the "opinion of the church" and which is not

This situation is different to the other two periods I mentioned.

I don't know if there were already times in history when we had this situation.

If no, the opinion of the church in earlier times is simply not comparable to the opinion of the church today because the situation of the church is a completely different one.

And the reason of the change of the opinion of the church is simply the change of the situation.

I'm focusing on the question why the opinion of the church changed in the past, not the "historic landmarks" (single events) that lead there.

I'd like to look on two periods in time:

Before AD 313 (edict of Milan)

The situation then was completely different than the situation today:

• There were many contradictory doctrines (example: Arian controversy) and unlike today there was no authority deciding which opinion was the "official one" when there were contradictory opinions.
  So we cannot say which one was "the opinion of the church" when there were both voices favoring and rejecting capital punishment.
  We know that many Christians in the first centuries saw military service as sin. Maybe they also saw capital punishment as sin.
• People working in the legal system were (nearly only) non-Christians. So seeing capital punishment as "allowed" does not necessarily mean that Christians are allowed to be involved!
  However the catechism (today) can only answer the question if Christians are allowed to be involved.
• Other forms of punishment simply were not known or they were not feasible in that time (thanks Thunderforge for your comment)

After AD 385

According to Wikipedia no later than AD 380 the Roman government started to influence teaching of the church in major aspects. No later than AD 385 Christians having a different belief even were executed.

This means that Christian teaching was massively influenced by people we would call "politicians" today.

In later periods this was even worse:

In the medieval period for example bishops typically also were politicians. You might find this "polemic" but unfortunately they often adapted Christian teaching to their political interests and not the other way round. (There are for example paintings from that period showing this.)

Today

Today we have a completely different situation:

• The most important: Christian teaching is not influenced by political or other interests (at least I hope so)
• If the church would say: "Capital punishment is allowed" this definitely would mean: "Christians are allowed to be involved"
• There are other methods of punishment which work well
• Although there are many different opinions in the Catholic church (as a Catholic I know this!) there is an authority defining which of these opinions is the "opinion of the church" and which is not

This situation is different to the other two periods I mentioned.

I don't know if there were already times in history when we had this situation.

If no, the opinion of the church in earlier times is simply not comparable to the opinion of the church today because the situation of the church is a completely different one.

And the reason of the change of the opinion of the church is simply the change of the situation.

Some notes about my edits:

I removed the term "early church" from my answer because (according to some internet findings) this term is understood differently by different people.

I also apologize for the statement in my first answer which was understood in a way that I claim that the church today and the medieval church are not the same organization.

The sentence was not meant the same way as: "Australia and Austria are not the same country" but more like: "The USA under Trump and the USA under Obama are not the same country".

By writing:

Clearly, many years ago, the church was in favor of the death penalty ...

This is a bit like comparing apples to pears.

Let's look at three different periods from Nathaniel's answer to see why:

Early church

According to the texts cited by Nathaniel the early church saw capital punishment as legal.

However you have to keep in mind that in the early church (very roughly up to the year 300) the civil authorities, politicians etc. were not Christian. In some times there was even persecution of Christians. Maybe the authors of those texts could not even imagine that Christians would ever be in an important political position.

In this situation those texts can be interpreted as: "God allows you to be killed by non-Christian people", but definitely not as: "God allows you to kill."

The catechism (today) however is addressing the question: "Are you allowed to kill?", not: "Are you allowed to be killed?"

These are two completely different questions.

Unfortunately I cannot tell you how early Christians would have answered if they were asked if capital punishment should be legal in a state or country governed by Christians.

However I can imagine that they would have denied this because I know that in the early church even military service was seen as a sin.

Medieval period, reformation era ...

In these periods (especially in the medieval period) the church definitely had the power to abolish capital punishment.

However in these periods the church definitely was not what you would call "a church" today:

It was a political organization having a lot of non-religious (political, financial, etc.) interests sometimes not even having scruples intentionally teaching things contradicting Christian religion to enforce those interests.

(I'm sorry if you find this "polemic" but there are some texts and - even more important for the medieval period - artwork like paintings showing that this kind of teaching did not only exist but that it was common in that period.)

If you have any text written in that times you'll have to consider that this text possibly does not reflect the Christian belief of that time but that it reflects those non-religious interests.

Therefore you cannot simply compare the opinion of the church then to the opinion of the church today.

Today we have the following situation:

In many countries Christians have the power to abolish capital punishment and the church is really interested in supporting Christian religion and not having any interests of higher priority.

This situation is completely different to the early church and it is completely different to the medieval period.

If you want to compare the opinion of the church today to the opinion in other times you'll have to compare to any other time when we had the same situation.

I doubt that you will find a lot of examples where the church was in favor for capital punishment in such times.

I think that the questions ...

• ... why the situation of the church changed so much during the time and ...
• ... if the Catholic church today and the Catholic church of the year XYZ really are the same organization or different organizations ...

... are both closely related to your question. However both questions are not discussed here and I fear that the answers to both questions would mainly be opinion-based.

I'd like to look on two periods in time:

Before AD 313 (edict of Milan)

The situation then was completely different than the situation today:

• There were many contradictory doctrines (example: Arian controversy) and unlike today there was no authority deciding which opinion was the "official one" when there were contradictory opinions.
  So we cannot say which one was "the opinion of the church" when there were both voices favoring and rejecting capital punishment.
  We know that many Christians in the first centuries saw military service as sin. Maybe they also saw capital punishment as sin.
• People working in the legal system were (nearly only) non-Christians. So seeing capital punishment as "allowed" does not necessarily mean that Christians are allowed to be involved!
  However the catechism (today) can only answer the question if Christians are allowed to be involved.
• Other forms of punishment simply were not known or they were not feasible in that time (thanks Thunderforge for your comment)

After AD 385

According to Wikipedia no later than AD 380 the Roman government started to influence teaching of the church in major aspects. No later than AD 385 Christians having a different belief even were executed.

This means that Christian teaching was massively influenced by people we would call "politicians" today.

In later periods this was even worse:

In the medieval period for example bishops typically also were politicians. You might find this "polemic" but unfortunately they often adapted Christian teaching to their political interests and not the other way round. (There are for example paintings from that period showing this.)

Today we have a completely different situation:

• The most important: Christian teaching is not influenced by political or other interests (at least I hope so)
• If the church would say: "Capital punishment is allowed" this definitely would mean: "Christians are allowed to be involved"
• There are other methods of punishment which work well
• Although there are many different opinions in the Catholic church (as a Catholic I know this!) there is an authority defining which of these opinions is the "opinion of the church" and which is not

This situation is different to the other two periods I mentioned.

I don't know if there were already times in history when we had this situation.

If no, the opinion of the church in earlier times is simply not comparable to the opinion of the church today because the situation of the church is a completely different one.

And the reason of the change of the opinion of the church is simply the change of the situation.

No, a polemic would not answer the question.

Sorry for having written my personal opinion in the comment.

But I would definitely deny a "dogma" to be the "opinion of a church" if it only represents the non-religious interests I have written about.

I doubt that you will find a lot of examples where the church was in favor for capital punishment in such times.

I think that the questions ...

• ... why the situation of the church changed so much during the time and ...
• ... if the Catholic church today and the Catholic church of the year XYZ really are the same organization or different organizations ...

... are both closely related to your question. However both questions are not discussed here and I fear that the answers to both questions would mainly be opinion-based.