Aoi, on the subject of casualness, I personally think that it's a good idea to have at least a little emotional impact, especially when killer and victim know each other. Not "What have I done?!?!?" levels, but more like, "Wow, I just killed someone I cared about. That was really intense, I think my hands are still shaking. ...and why am I crying?"

I will mention that's sort of the feel I'm going for with the Nanoha fic Eterya and I are working on that's based on your Eva idea. It's still mostly casual, they're ending their lives and don't see anything wrong with it, but a fair majority of the kills are going to leave the killer somewhat emotional just from the intensity and intimacy of the act.