Betrayal. [ Those were simpler times, really ] Oaths were sacred, so was hospitality. If you killed someone in a drunken brawl, eh you could pay that up, make sure to take care of his kids if he had any and so on.

[ Accidents happen. Sometimes those are not accidents, and maybe you just hated each other and decided to deal with it like good adults - prove yourself in a duel. Endless possibilities there ]

But if you broke your promise, or attacked someone under a friendly roof, you were as good as worthless.