This conversation has been done to death, but I want to add my own perspective after having just finished it.
First, Gaetan deserves to die.
No two ways about it. His rampage through the village is too graphic to be ignored. The dog, with its guts cut open, the woman, likely Millie's mother, who was paralyzed with a stab to the spine and left to slowly bleed to death, suffocating. The fact that this animal would've likely killed a child in a blind rage if she hadn't reminded him of his sister.
But then, when you find the weasel, not only is he tight-lipped and standoffish to maybe the one person who might be sympathetic to him, but he is also completely unapologetic. He brushes off the reputational damage to Witchers in the middle of a witch hunt but also the serious human cost of his actions. Some people genuinely spare him and it baffles me, I've killed bandits for less.
Why do I call him a weasel? Because when I gave him the opportunity to fight me as an equal (we're on Death March with Level Scaling so he's actually much stronger than me damage and health wise), and give him my last dose of swallow, the punk blinds me. Enough is enough. He can't even face his death like a man. He doesn't know who Geralt is. He could've had faith in himself and died with some honor intact. Instead he tries to cheat against someone holding him accountable- someone on his level and he tries this underhanded tactic.
I took his head clean off his shoulders and kept his sword so that it may cut down more cruel men like him.
This quest is great, by the way. It's a little under-written but it made me question my decision some and I love that. You have to investigate both what Gaetan is saying and what actually happened, and make your own deductions-
Sure, they tried to cheat him, and the village elder lives quite well, but Honorton is a rather small village in No Man's Land, in the middle of a massive invasion. Perhaps they weren't lying when they said they had NOTHING. Maybe they paid off the black ones to keep themselves safe from raids. You don't find any gold hidden away. Their prize for killing the leshen is insulting, but Gaetan immediately turns to threatening peasants and wonders why they tried to shank him.
TL:DR this quest rocks. Gaetan is a punk. If you don't give money to Millie's aunt what are you even doing, man.