Oh don't go there if you are talking about the past century.
Violence has to do with ignorance, lacking of education and tolerance, that goes without saying but what's the use of religions if they can't free their followers from ignorance? In fact, because religious scriptures are full of vague texts, they can be interpreted in the most fallacious ways (on purpose) to feed the naive, uneducated, ignorant parts of the societies with. That's the problem we have in Iran; where violence could be easily justified through religious beliefs. Tragedy happens when the government in charge prevails/rewards this but you know where the question lies? Why such corrupt ideological government can exist in the first place? Isn't it because religion (Islam in this case) prepares the ground for it?
My problem with majority of posts here however is that people tend to hook onto this "religions are the roots of today's turmoils" mindset quite easily. Religions are good for the most part. Religions can make people have better lives, live healthier, do more good. It's not an absolute remedy (what is after all?), but a certain dose of rationality and education will keep people from falling for those wrong interpretations of religious instructions.
Nothing is considered worse in Islam than taking a life, but you can always take a verse and interpret it out of context, then you can easily lead the blind to violence under the name of Islam.