To attribute the actions of this POS to a possible physical problem is to ignore and dismiss the whole concept of evil. This guy did not do this because he had a "mental" illness. He did it because he was a souless, heartless monster. He did it be HE (his entire being, not merely his brain synapses) was evil. How he got there is not clear but somewhere along the line he ceased being a human being and became an evil, heartless and soulless monster.
No one can kill a room full of children because of some chemical imbalance or imperfect brain activity. If you kill a room full of children it is because you are a evil monster.
Evil exists my FRiend. It is not mental illness. It is evil. If you fail to recognize it, you may become a victim of it.
These children all saw it before they died. It tears me up inside to know that the last thing they saw on earth was an evil heartless soulless monster. No amount of mental health could have stopped this because it wasn't a mental problem. He may have had some underlying mental issues (don't we all) but when you cross certain lines it is no longer a brain issue, but a soul issue. This guy had no soul.
Some people are beyond help and this psycho was unfortunately one of them.