Genesis 3-5

I needed about 15 minutes to read this passage. Cain murdered Abel is a jealsous rage. The God of life put a warning mark on Cain. A mark that indicated a curse with a curse - do not kill this man to avenge Abel. Everyone must have known Cain and Abel. This first generation removed from the garden couldn't have been too numerous. Maybe even a second and third generation had been born. If everyone knew the brother they also knew the story. Nothing like this had ever happened. So why did God not require life for life? Why did God protect the man who sought to be like God; ruling over life itself?
I don't have any idea right now.
Stuart