Abel and Cain were going to make a sacrifice to god on demands of their father Adam. Cain was going to kill Abel out of anger of his Brother's sacrifice being accepted despite him being a mere shepherd. God spoke to him about his impure and malicious thoughts.
Cain
Don't do something you will regret. Your brother has done nothing to you
Cain carried on with his plan anyways and murdered Abel with a rock. Deeming him the world's first killer. Cain was cursed with "The mark of Cain" and could not be killed by anybody else.
Cain
Abel
We all have anger and jealousy towards someone at one point or another, whether it be friend, family, or even a stranger. Instead of directing this jealousy into hatred and using it against that person, we should learn to have faith in God and think about the consequences of your actions. God sees all sins, and will punish you for them if you don't repent. Even Cain was protected by god and had a second chance. Whoever were to kill Cain would be suffering 7x more despite the fact that Cain murdered his brother. When I was younger, I used to get mad at my little Brother and would either hit him, albeit not hard, but there were times where I would get frustrated. I acted upon emotion and jealousy and that led to me getting a lecture on why I should not act in such ways.
"But the Lord said to him, “Not so ; anyone who kills Cain will suffer vengeance seven times over.” Then the Lord put a mark on Cain so that no one who found him would kill him."