According to Michel, they were told that the initiation was justice for three elves who had "done injury" to nobles the previous year. But he says there was no indication who these elves were. It was just a pretext.
If we ever have a chevalier as a companion, it better be a repentant ex-chevalier.
I'm genuinely torn about things like this -- on one hand, chevaliers have interesting lore and history behind them. On the other hand, it looks like they're pretty much among the most selfish, cruel people in Thedas. It would be interesting to have them as a party member, but I don't know if I'd want them whitewashed either.
It would be interesting to have a chevalier who is awful enough to actually make the "no, I don't want you joining my party" dialogue actually viable. Though I don't know if it's worth developer time creating a character that people will actually want out of the party, or who will strongly clash with any decent character in the party. So maybe it's conceptually interesting, but no reasonable in practice.
On the other hand, a repentant ex-chevalier is possibly the only practical way of doing it. It would allow them to join the party without being pretty much the worst, and it would provide justification for it to be a specialization.