Well firstly I'd point out that Islam is a much younger religion (^c 600 years IIRC) and so is currently at the age Christianity was in the Middle Ages, and what Muslims are doing today pales in comparison to what Christianity did back then to secure its place as a global religion. When Christianity was around the age Islam is it was all about Crusades, Inquisitions, Reformations, Colonialism, Witch hunts and burning heretics.

And if you want examples of Christian extremism in modern times: KKK, ethnic cleansing in Central African Republic, bombings of abortion clinics and gay/lesbian night clubs in USA (Plus numerous right wing acts of terrorism are perpetrated by Christians with a grey area as to whether they are political or religious in motivation), religious based war crimes during the split of the former Yugoslavia, Eastern Lightning activities in China, National Liberation Front of Tripura in India, 'The Troubles' in Northern Ireland, Maronite Christians in Lebanon, the Russian Orthodox Army in Ukraine, Christians burning Muslim schools in Indonesia, Ilaga is a Catholic Extemist group still active in southern Philipines, the Lord's resistance army in Uganda.

Basically anywhere where Christianity is a minority religion its adherents still engage in bloodshed to promote it, we just see less of it in the west because it is already so well established here. Christianity set the precedent that if you want a Middle Eastern cult to become a global phenomenon you better be prepared to do some killings... Islamic extremism is just following this tried and tested method.