This Bible contradiction is from the EvilBible.com.
The blood-shedder must die
Genesis 9:5-6 View context
And surely your blood of your lives will I require; at the hand of every beast will I require it, and at the hand of man; at the hand of every man's brother will I require the life of man.
Whoso sheddeth man's blood, by man shall his blood be shed: for in the image of God made he man.
The blood-shedder must not die
Genesis 4:15 View context
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.