As a Christian, I believe that God has everything planned out. Whether someone is mean or nice to you, you do not have to do anything, God will take care of it. In a way, I suppose it is karma. However, I believe that we're all inherently sinners from Adam and Eve, so there are no 'good' or 'bad' people, as we are all bad and have free choice.
(Please don't come after me, btw. This is my religion. You can disagree, but do it respectfully.)