What did Waymar Royce do to be sent to the Night's Watch?

Question

Waymar Royce was the head of the party of three to investigate the rumors of White Walkers. Why did he join the Night's Watch?

Answer 1

Waymar Royce didn't do anything, he joined of his own free will:

As a third son of a lord, Waymar did not have many chances at wealth or land and so joined the Night's Watch.

Answer 2

I felt like simply the fact he was a third-born son wasn't enough explanation on its own for why he'd willingly take a fate usually reserved for rapists, murderers and other dishonoured people, so I read up a little more on House Royce (after all, most third-born sons don't do that: among Starks its seen as honourable, but very few others see things the same way, and the Royces aren't from the North, they're from the Vale).

Here are some relevant points as to why the Royces might have a more positive attitude to the wall, closer to that of the Starks than to other non-Northern families:

• They "trace their blood to the days of the early First Men". In general, it seems like families with stronger cultural and genetic roots to First Men take the duty of defending against the horrors of the far north more seriously than families who are more Andal. Most unexplained watchmen are from families with strong first-men roots (e.g. Benjen Stark, Jeor Mormont)
• Despite not strictly being Northerners, they seem to have close historical ties with the Starks:
  • "Kyle Royce was a member of Brandon Stark's party when he went to King's Landing to demand Prince Rhaegar Targaryen's head for kidnapping his sister, Lyanna Stark"
  • "During the War of the Five Kings, Lord Yohn [Royce] is one of the lords of the Vale who wishes Lady Lysa Arryn to ally with Robb Stark"
• They also used to be kings of their region before the Andal invasion. "The house is thought to have started in the Age of Heroes, though supposedly, their history goes back as far as the Dawn Age", which would include the Long Night i.e. when Others invaded before the wall was built.

So while it's not stated explicitly, it seems like they're quite culturally similar and historically close to the Starks, and most likely also have (ancient) history from the last times trouble came from beyond the wall.

This would explain why they might have a comparatively positive outlook to the Watch as being a solemn duty more than a punishment, and this fits how Waymar Royce himself was presented - as someone who, in the eyes of the watchmen who were there as a punishment, took his role a little too seriously, with little patience for those who wanted to do the minimum then go home.

Answer 3

In a Feudal society, for every Lord, there are heirs and then there are spares. The Eldest son is the heir usually and the second son is kept around as a backup in case of the heir dying, although in many houses Second Sons are considered just as spare and this is precisely the reason why the mercenary company Second Sons exists.

Waymar Royce was the third son of Bronze Yohn Royce. He had absolutely no chance of inheriting Runestone so he had to make his own destiny. He could therefore:

1. Become a Maester
2. Become a Septon
3. Be a mercenary
4. Serve his eldest brother as a Household Knight
5. Join the Night's Watch

Waymar doesn't appear to be the scholarly kind so he couldn't have joined the Citadel. He presumably doesn't follow the faith so becoming a Septon is also out of the question, given his first men blood. But that is a weak argument, he could be a follower of the Faith given that he is a Knight. Still he doesn't seem the scholarly or pious kind to consider this. A mercenary is still a viable choice but it lacks honor, something ancient houses like House Royce are very concerned about. He could have served his brother like Kevan served Tywin but he had little chance of progress. Night's Watch however, he could rise high there. He could be commander of a Castle or be the Lord Commander eventually. Moreover, It is considered an honor to serve the Night's Watch, among the First Men at least and House Royce is of First Men Stock.

Ser Waymar Royce was the youngest son of an ancient house with too many heirs. He was a handsome youth of eighteen, grey-eyed and graceful and slender as a knife. Mounted on his huge black destrier, the knight towered above Will and Gared on their smaller garrons. He wore black leather boots, black woolen pants, black moleskin gloves, and a fine supple coat of gleaming black ringmail over layers of black wool and boiled leather. Ser Waymar had been a Sworn Brother of the Night’s Watch for less than half a year, but no one could say he had not prepared for his vocation. At least insofar as his wardrobe was concerned.
_{AGOT - Prologue}

Given that it is said that his house had too many heirs, We can assume that his elder brother Andar had fathered his own children, making both Robar and Waymar even more useless as spares. Robar Royce set out to forge his own destiny as well but he had different ideas than Waymar:

"You are a long way from the Vale, ser,” she told him.

“And you far from Winterfell, my lady.”

“I know what brought me here, but why have you come? This is not your battle, no more than it is mine.”

“I made it my battle when I made Renly my king.”

“The Royces are bannermen to House Arryn.”

“My lord father owes Lady Lysa fealty, as does his heir. A second son must find glory where he can.” Ser Robar shrugged. “A man grows weary of tourneys.”
_{ACOK - Catelyn III}