Yes, Christians did some barbaric things and some other Christians still do barbaric things. Just like some Buddhists do barbaric things and Atheists do other barbaric things.
You can't judge a band by its fans and instead must judge ideologies by their book. Or do we criticise Democracy because democrats have committed genocidal acts? The Quoran clearly advocates the killing of unbelievers who "mistreat" Muslims, with a broad spectrum of political-religious rulings that define how this "mistreatment" looks like. Not to mention the fact that Islam is an ideology that defines government style, law and regulates all affairs of day-to-day life.
Since I've already addressed this point today, here:
http://www.christianforums.com/t7836404-2/#post66138480
http://www.christianforums.com/t7836404-2/#post66138974
http://www.christianforums.com/t7836404-2/#post66138974
Christians are bound to follow the commandments set forth in the New Testament.
Tell me, where in the New Testament are these commandments you speak of?
You can criticise the violent history of Christianity and I find your points legitimate.
But you cannot get away with equating Christianity with the perverse ideology that the Islamic State is following.