You have heard that it was said, "You shall love your neighbor and hate your enemy."
But I say to you, love your enemies and pray for those who would persecute you, so that you may be like your Father in Heaven; for He makes His sun rise on the evil and the good, and sends rain for the just, and for the unjust.
Aside from forcing one to interrupt one's train of blame, this precept is hardest to apply when most needed; and then it's a really great idea!
It isn't just 'doing good in return for evil,' but sincerely wishing good for the other! What a stretch!
What makes this perhaps the best passage in the Bible, is the description of God -- which makes drivelling absurdity of what used to be the most popular interpretations of Christianity. This is how to be 'children of God' because this is what God is like! It's necessary to throw out the consolations of Hell, because God might occasionally change tactics -- but not His basic intention: good for each and all, whether they themselves are 'good' or otherwise. That may need to be 'different strokes for different folks' in the bad sense, but His ultimate purpose can't be frustrated forever.