Islam and Christianity have an interesting contrast. Jesus never killed anyone, and Christianity took 300 years before the killing began.
For the first century of its existence, Islam was absolutely soaked in blood. The killing only slowed down as the Islamic empire finally ran into boundaries in the 8th century, after about a century of expansionist, imperialist, unprovoked Islamic aggression.
Even after the initial expansion slowed, the killings did not end. Slaughter (jihad) and oppression (sharia) are part of the core doctrines of Islam. Killing for Islam is not a modern idea, and it will never end until some sort of reformation takes place within the religion. Medieval Christianity was equally violent, but Christianity has since reformed.
For many years now, Islam has been the most violent religion in the world.