The Word of God tells us that we must not begin by looking at outside influences, not even at the chemistry of the brain. What happened is an example of the sinful depravity of a human heart. The problem does not find its solution simply in a few isolated events or a few isolated people. The Word of God teaches us that all are fallen into sin and that sin is depravity and that man's problems do not begin from outside of himself but are to be traced within. The Scripture states the problem correctly, it states it honestly and forthrightly, that man's problem is the depravity of his own heart.
Man is a fallen sinner.
Mankind and the human heart are as depraved as they can be. The problem is sin, sin which began already in Paradise when Adam transgressed the command of God, sin which brought devastating consequences. The first parents, Adam and Eve, were grieving parents. Cain, their son, killed their son Abel, his brother. Especially when we are spurned, when we are ostracized, there is a reaction.