Because there's a very large chance of killing someone other than who you're trying to (if that target is one you're "allowed" to kill in the first place).
Because, conceivably, the former scenario might involve the person trying to kill you. I've heard no reports of someone committing homicide in their sleep.Same reason we consider it more acceptable to fight someone rather than stab them to death in their sleep.
And yet I just quantified it. Morality wasn't determined by flipping coins.It doesn't have to make sense. It isn't quantifiable. It's morals, and poison is immoral.