Laws are a social construct. Law is not.
If you're talking about "Law" in the Capital L sense, then you're referring to the same thing as me, when I mentioned people's values/morals/philosophy/etc.. No one has a complete handle on the absolute expression of it, but people do make an effort on their own, outside the "social constructs". In which case, I would say many aren't simply just delivering "vengeance" as you said, but actually making an attempt at sound judgement, as best they can.
Either way, it's up to us. There's no council of law mechanic in the game. The developers intend everyone to make whatever choice they want. It's right for us to judge because they gave us the choice...and they are the gods and demigods of TES, and they've endowed the player with the power to make the call. There's no morality meter. Whatever choice you make can be rationalized as right, depending on the player and roleplaying. If it was Mass Effect or D&D, certain choices would make you "evil", but thank the gods such a thing doesn't exist in this game.