Why are Christians moral absolutists in regard to non-believers but relativists in regard to their own sins?

This is a straw man. When you become a Christian you begin a new life and you put your life og sin behind you and each day draw closer to God and become more Christ like and sin less. Being a Christian is NOT an excuse to sin more. People who sin when claiming to be a Christian are either immature Christians or most likely not saved at all. Take the example of the inquisitions or slavery - this was carried out by a load of "religious" people who didn't reflect the life of Jesus very well and I seriously doubt were saved anyway. We don't need religion, we need Jesus!