Cain's life was a wild jungle with a vicious beast crouching at the door of his heart, and it wants to take over. When we're in this situation, we've got to look hard at what's wrong and get the help needed to survive. However, much of the time we bask in some exalted sense of self-approval. He was on the wrong road and refusing to listen to God, he decided to fight back. Part of what caused the death of Abel was jealousy.
People treat others badly because they are unhappy with their lot in life.
Unhappiness is not the fault of the person you're fighting with. The reason you're unhappy is that you won't humble yourself and ask God for what you need. The ultimate reason for friction, anger, violence, jealousy, coldness, and rage in society, among nations, in families, between us and the person who has treated us the worst and whom we have treated the worst, is an essential anger with God that he has not given us what we want. So we strike out at people around us. There are several reasons we have trouble renouncing sin and embracing righteousness.