I work on a "If they're still a danger" system.
The Alarune i spared because she basically devoted herself to me and agreed to make her effigies stop harassing the locals and work in my wayfort instead.
The Centaur Behemoth i killed because it's rage and anger posed a risk to others, and killing it felt more like a mercy.
Queen Alissa was killed because she didn't even seem a little bit repentant, or at the very least had caused so much pain and damage that sparing her would only torment her victims further.
Centaur marauders and Imp leaders are always killed, because if i didn't kill them, then they'd just go right back to hurting others.
I probably would have killed Jael'yn too if there was an option for it. I sent her to the Frost Hound to prevent her from turning pure demon, but I'd prefer if i could keep her at the wayfort just to keep a closer eye on her, and i feel like Garth isn't too fond of having her around.