Adam and Eve’s first two sons were Cain, who grew plants, and Abel, who raised animals. They offered some of their plants and animals to God, but God only liked Abel’s meat, and didn’t like Cain’s fruits. Cain got very angry, and the all-knowing God couldn’t figure out why.
Then jealous Cain killed Abel, so God cursed Cain and didn’t let him grow plants anymore. But God also put a mark on Cain, to warn people not to kill him. If anyone killed Cain, God would kill that person seven times, or something.
Later, Cain’s great-great-great grandson Lamech, encouraged by what had happened to Cain, also killed someone. Lamech decided that if anyone killed him, God would have to avenge him 77 times.
The moral of the story
If you don’t want to be killed, you should kill somebody else, and then God will personally protect you for the rest of your life.
(Disclaimer: I’m not saying killing people is actually a good idea. That’s just what the Bible says. The Bible is wrong, as usual.)