You conveniently forget that what people have done in the past isn't necessarily what Christ told them to do. Violence is quite strictly against Christ's teachings, although the grim reality of the world has always forced people to either fight or perish. (The conflict between killing and Christianity's teachings was, obviously, a well understood dilemma, and many knights and warriors were genuinely concerned about their soul's salvation.) Whereas Islam is entirely built on violence, jihad and conquest. Islam's fast expansion in the 7th and 8th century was greatly aided by the mere promise of loot and plunder to the warriors of Islam, causing many warlike tribes to join it for that specific reason.