The most difficult math problem in the universe, it turns out, is 70 x 7. Perhaps the hardest thing to do in the Christian life is to forgive someone who has hurt you, often badly. But Jesus says the alternative to forgiving one's enemies is hell.
One of the reasons this is hard for us is because we too often assume forgiving a trespasser means allowing an injustice to stand. This attitude betrays a defective eschatology. At our Lord's arrest (Matt. 26:47-54), Jesus told Peter to put his sword back into his sheath not because Jesus didn't believe in punishing evildoers (think Armageddon). Jesus told Peter he could have an armada of angelic warriors at his side (and one day he will). But judgment was not yet, and Peter wasn't judge.
That's the point.