Killing a human, I think, would push John too far over the edge, likely past the point of no return. You saw how he almost lost control completely when he burned down the building and killed the cat; if that was another human, his rules would have all been tossed out the window. Break 'em once, it becomes easier to break them again, and again, and again.
And this would be true even if the human in question was a killer as well (not to mentioned if John began killing human killers, he may as well just move to Miami and change his name to Dexter). He is capable of killing a human; the question is, is he capable of killing one and not letting Mr. Monster take over completely? The demons, in a way (at least this is my theory) represent John's own personal demons. He can kill them and it fulfill the bloodlust of Mr. Monster, but keep himself human (however much of him is a still "human") and keep following his personal code.
Opposing and killing the 'demons' is the same as facing and repressing his own inner demon - which is why when John almost gave it to join the torturer in Mr. Monster, it would have been the same as giving in to his own demon. And from there, it would just be a hop, skip and a jump away from killing a human and becoming that which he is trying to avoid.
I don't know, I thought the 'demon' was well-foreshadowed simply by the fact that there was a 'demon' in the first book. I thought the monster is this book was amazing and horrifying in ways that Crowley wasn't capable of.