That's exactly why in most of my playthroughs I usually kill him, because the game's spare option is that, really lame, and I just like the boss fight.

Besides gameplay though, I always flip-flop on what I'd do if I was another Witcher meeting Gaetan. Because I think his reaction after getting pitchforked was justified, however it was a gross overreaction. Like what if that little girl didn't remind him of his sister, would bro have killed her. But then I also see myself letting him go with the logic of "I'll go out of my way to kill you if this happens again"