When can I start not killing people? Right in the very beginning as soon as I exit the cell? Or can I kill those people since it was a tutorial? I want to go for a pacifist/stealth run and don't want to kill anyone I don't have to. The game isn't making it very clear when to begin and I accidentally killed two guards.

As soon as you leave your cell, you can't kill anyone.

I figured that out, but thanks. I didn't want to do the whole level over again. I just knocked them all out this time.

Yeah that happened to me it didn't even realize I killed someone in the prison area restarted it was good but didn't ghost I did the first time. The first mission said I killed someone pretty sure I didnt but I guess bodies can fall off things and stuff, but it is ok only first playthrough.

I think I'd better stop reading this forum now, I already know too much.

Maybe the rats got your unconsious guards? Better put them in a safe spot far from the ground