2 suggestions.
I am against the idea of killing the guards, but there should be a way to distract/move them.
I also think instead of making it so that you cannot exit the level when the alarm is up, I think sentries should rise out of the floor or walls to restrict movement. There could be sentries right next to the escape if you don't want people leaving, or you could set them up on one side to make them take a specific route.