Who slew Cain?

No one killed Cain, but Cain killed Abel. God put a mark on Abel's head so that no one would kill him.

Genesis 4:15

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.

Cain probably just died of old age.