A while back a professional basketball player had his contract terminated because he physically attacked his coach. In his response to the suspension the man felt the NBA was overreacting. And then, he sued his agent because the agent didn't get him a contract that could not be terminated. There was no sincere repentance for his act. Like Cain, we deserve far worse than we get.
Sin carries a stiff penalty - death.
If God were to act in justice rather than mercy we would die the moment we sinned. But God extends mercy to us because he is going to exact justice through Christ. The account of Cain and Abel in the Bible is one of great saddness. The dreams of parents are shattered. It began with the dream that their son would be the promised deliverer.