Genesis 4:15

And the LORD said unto him, Therefore whosoever slayeth Cain, vengeance shall be taken on him sevenfold. And the LORD set a mark upon Cain, lest any finding him should kill him.

1. The LORD took steps to protect Cain from being killed, promising a much worse punishment for anyone who did.
2. The LORD set a mark upon Cain to make him readily identifiable and prevent him from being killed by mistake.
3. For some unexplained reason, the LORD really seems to have wanted Cain to live a long life.