I don't think he was a "monster" by choice, you really should get yourself educated over psychiatric matters.
There are mental illness and disorders and a lot of them do not disappear by just saying : " Oh ,go walk a bit and all your problems will go away."
So as brutal and dark the outcome of the guy's mental illness is, he is not to be blamed.
It is the society around him (Friends,family )job to have reported about him if he acted unusual.
But most don't, that's when the mental patient just goes off the edge, especially when not taking medication or under special supervision.