When Jesus tells Peter to forgive 70 times 7 (or 77, depending on translation), He is countering the vengeance in the story of Cain and Lamech.
Then Peter came to Jesus and asked, “Lord, how many times shall I forgive my brother or sister who sins against me? Up to seven times?”
Jesus answered, “I tell you, not seven times, but seventy-seven times.
Matthew 18:21-22 (NIV)
Lamech said to his wives,
“Adah and Zillah, listen to me;
wives of Lamech, hear my words.
I have killed a man for wounding me,
a young man for injuring me.
If Cain is avenged seven times,
then Lamech seventy-seven times.”
Genesis 4:23-24 (NIV)