There has been a bunch of interesting answers already, but one thing yet to be mentioned is the dying society.

So far all answers has been about trying to imagine stable, or even growing to thriving societies, but your question does not exclude societies that are neither stable nor prosperous.

If the main reason for not allowing people to tolerate the normalizing of killing is essentially because it will inevitably lead to the collapse of that society, then what if said society is already collapsing?

This concept is being played with in the RPG world of Mörk Borg, where everybody already knows that the world is ending. Doomsday is coming and everyone will inevitably die, so who cares if someone died just a day earlier.

This is interesting in many ways since people become more concerned about HOW to die, rather than not dying at all. There is for example a King in this world that prepares all the citizens in his domain for a huge mass suicide. The population is of course aware of this but doesn't even mind it. Some might even find the kings endeavor to be admirable, however the religious cult in this world considers suicide to be heresy and believe that doomsday should be welcomed, not avoided.

A world where people is mainly concerned about how to die rather than not dying gives room for a lot of interesting and gruesome conflicts between groups and people in the world. Blood is unavoidable when everybody rather sacrifice their life for whatever cause rather than face doomsday. It can lead to some great storytelling.

The way I see it, after reading the answers there is basically 3 options for the shape of such societies:

1. There are some sort of limitations or rules on who can kill who, how someone can be killed, or why they can be killed, alternatively some sort of quota. As long as these limitations or rules are followed nobody will raise an eyebrow over these killings. This is how most human tribes worked historically, but in worldbuilding we can take it to absurdum.

2. Dying is not the end, or alternatively people don't die at all, essentially making them undead i guess. In this scenario people are either easily resurrected, reborn, spawned or whatever, or simply can't die to begin with. Cutting someones head of might be done simply because it's hilarious, or even for practical reasons when you want to take a close look at something in a confined space not having enough room for your hole body etc.

3. My addition to the conversation. Society is collapsing so killings are normal, or even worse, as with the case in Mörk Borg, everybody is already doomed and forced to face a horrible death. The closer to unavoidable death, the more normalized it becomes.

There has been a bunch of interesting answers already, but one thing yet to be mentioned is the dying society.

So far all answers has been about trying to imagine stable, or even growing to thriving societies, but your question does not exclude societies that are neither stable nor prosperous.

If the main reason for not allowing people to tolerate the normalizing of killing is essentially because it will inevitably lead to the collapse of that society, then what if said society is already collapsing?

This concept is being played with in the RPG world of Mörk Borg, where everybody already knows that the world is ending. Doomsday is coming and everyone will inevitably die, so who cares if someone died just a day earlier.

This is interesting in many ways since people become more concerned about HOW to die, rather than not dying at all. There is for example a King in this world that prepares all the citizens in his domain for a huge mass suicide. The population is of course aware of this but doesn't even mind it. Some might even find the kings endeavor to be admirable, however the religious cult in this world considers suicide to be heresy and believe that doomsday should be welcomed, not avoided.

A world where people is mainly concerned about how to die rather than not dying gives room for a lot of interesting and gruesome conflicts between groups and people in the world. Blood is unavoidable when everybody rather sacrifice their life for whatever cause than face doomsday. It can lead to some great storytelling.

The way I see it, after reading the answers there is basically 3 options for the shape of such societies:

1. There are some sort of limitations or rules on who can kill who, how someone can be killed, or why they can be killed, alternatively some sort of quota. As long as these limitations or rules are followed nobody will raise an eyebrow over these killings. This is how most human tribes worked historically, but in worldbuilding we can take it to absurdum.

2. Dying is not the end, or alternatively people don't die at all, essentially making them undead i guess. In this scenario people are either easily resurrected, reborn, spawned or whatever, or simply can't die to begin with. Cutting someones head of might be done simply because it's hilarious, or even for practical reasons when you want to take a close look at something in a confined space not having enough room for your hole body etc.

3. My addition to the conversation. Society is collapsing so killings are normal, or even worse, as with the case in Mörk Borg, everybody is already doomed and forced to face a horrible death. The closer to unavoidable death, the more normalized it becomes.