1. Violence is not bad.
People use the word to mean bad. It's not. There is a great deal of difference from violently jerking your child back from on-rushing traffic and setting a bomb that kills children. The key modifier is violence for what? To protect the innocent and weak: violence is good. To cowardly hurt weak and innocent people to advance your wrathful cause is bad. There is no equivalency between the two, and that is why I have come to reject pacifism.
Therefore, when you ask, which religion is the most/least violent, you are not asking me which is the best. Catholic theology asks, violence for what? Augustine developed the Church's official war position in the early 5th century, which came to be called the Just War Theory. (Though I haven't read all of the article, Wikipedia's seems pretty accurate.)
2. What is religion X?
People have a lot of problems with this one. Not a day goes by when I don't see someone saying that Deuteronomy's laws are part of Christianity because it's in the Bible. People who say things like that don't understand that, beginning with Jesus (our founder) we interpret the OT, we do not literally follow it. If you think, well, that's dumb, you can't do that - well, that is what we do. Like it or not. So I would invite you to read Matthew 5:
21 “You have heard that it was said to the people long ago, ‘You shall not murder, and anyone who murders will be subject to judgment.’ 22 But I tell you that anyone who is angry with a brother or sister will be subject to judgment. Again, anyone who says to a brother or sister, ‘Raca, is answerable to the court. And anyone who says, ‘You fool!’ will be in danger of the fire of hell.
Okay, so that's our religion.
But, again, we are not a book religion, we are a Jesus religion and a Church religion, which in regard to the latter means that we consider our history determinative to some extent. What our great teachers, like Augustine, Aquinas, Gregory the Great, etc., taught is how we interpret the Bible. I think the same can be said for the great Jewish rabbis and the interpretation of the OT. So, if you want to know a religion, you can't just ignore what that religion says are its authorities.
As for Islam, I am sure they have an interpretive tradition too. And I am positive that the terrorists do not embody it. How do I know that? Because they didn't have explosive vests in the 7th century. This is not to say that Islam is not bent on world domination. I believe it is. Whereas Jesus was not interested in having a "Kingdom in this world," Mohammed actively fought to make one. That's a significant difference.
History is a messy thing. What Catholics have done, is not our history. We look to Augustine and Aquinas, not to Hitler and Frederick Barbarossa - although all four were baptized Catholics.
The fact is, the bishops and popes have always opposed the excesses of 'Christian' rulers. If you don't accept that, then you don't know history. St. Ambrose punished Emperor Theodosius for the Thesaalonika Massacre... so, who's the Christian - Ambrose or Theodosius?
Blessed John Henry Newman once made the cynical remark that Christianity only has a significance for the very few. I think it has some significance for many, but, yes, primary significance for the very few.
The fact is, I believe Christianity has had a positive impact on the world. Is it just a coincidence that the scientific revolution, universities and hospitals sprung up in Christian lands first? Is it a mere coincidence that slavery was abolished in Europe centuries before anywhere else, and that American abolitionists were always strong Christians? Its temerarious to deny this link.
4. Human freedom and the role of religious identity.
"Look at what these Christian did over there!" is meant to be a powerful indictment of our religion. It is not. It is an indictment of people who fail to follow Christian teaching. And, desperate times lead to desperate acts. I don't know what it's like to a Christian living in Africa of the Middle East under the threat of Islam. I don't know what I would do in those circumstances. But I would not blame Jesus for what I chose to do.
Christians know when they are being Christ-like and when they are not.
In the end, Christianity is a religion of love and Islam one of submission. I think how these two priorities have affected their natures and histories is fairly evident. Jesus spent His life trying to get people to love; Mohammed, trying to conquer territory for God.