He's the king, and his own adviser doesn't bother bowing down to him, not to mention that she's a plotter that plots the deaths of kis, what the hell did you expect? Personally I thought he was being rather merciful.
Good riddance, sorcerers, sorceresses, scheming, self-centred, narcissistic untrustworthy bastards, the whole lot of them.
Even Triss, or rather especially Triss. "Sile is responsible of the king slayers..." Uh, just Sile? What about the rest of the Lodge, the head conspirator Phillipa included. Oh, right, that would include you, right.
And then the other line of her after the fight with dragon is over, "Sorcerers need their place, or another Lodge is inevitable." Ah, the one you joined, right. Because they will conspire to bring instability, bring death and terror, we must bow to their demands and hand them all manner of privileges and funding to research ever greater magic and have complete control over who sits at the right hand of a king, making them essentially above the kings? No, that would be reasons kill the lot of them and be done with it.
God, I hate Triss, I hated her ever since her conspiring communication with the rest of the lodge in Witcher 1, and Witcher 2 only lowers my opinion of her. Wish she was right alongside the rest of the magic terrorists burning on the pires.
PS: As for who started the Witch/Warlock hunt, I'm pretty sure Iorveth said after I killed the dragon it was the Nilfgaardians that started it, and the north's armies then gleefully joined in.