You'd change it to not ask you if you're not expecting to use it during the fight.
but
then remembering to toggle it off is a bit of a hassle.
Even if you knock someone down by accident you can just finish them off.
Actually my recent encounter with Hag was hilarious. When she changed into Mayrina I toggled non-lethal just so I won't kill the girl by accident - I ended up "knocking unconcious" the illusion and the effect tranfered over to Hag herself - who had still just under 100HP left. So she just lied there unconcius as my noble paladin beat her to death. Seriously, I need to work hard in this game to accidently not cheese it.
