I was gonna sleepdart Callista, or possess her - while she bathes. But it kills her for some reason. Clearly a bug?

I don't wanna know what you were planning to do !

( and I don't know, sounds like a bug, yeah. Sleep darting her in her bath may count as a kill, with her drowning and stuff... right? )

The game doesn't have a system where mouth + water = drowning. Any character who's body is in water and is unconscious dies. Even some puddles can kill guards.

Also it doesn't make a difference if you kill or tranquilize a member of the Loyalists...