But, but, but ... Christianity is different! The Crusaders who slaughtered Jews in Medieval-era Germany weren't "really" Christian! The Spanish Inquisition officials and executioners weren't "really" Christian. The Protestants and Roman Catholics who bombed and shot and committed other violence in Northern Ireland weren't "really" Christian. The members of the KKK aren't "really" Christian.
(Yes, I've heard that argument, sadly.)