I disagree that they aren't all dangerous.
http://www.usatoday.com/story/news/nation/2015/03/21/new-orleans-airport-attack-scene/25136201/
The KKK was partly Christian in origin and had some goals regarding Christianity:
http://en.m.wikipedia.org/wiki/Christian_terrorism
Also on the above link are other examples of Christian extremism.
Have you read about these wars?
http://en.m.wikipedia.org/wiki/Crusades
You give me enough time and I'll find a huge list of proof that ANY theism is dangerous. If a person does cocain, gets drunk, or par takes in some other chemical dependency; what are the chances that the dependancy might make them harm innocents? The chances exist, but if they do it's likely to be one target in particular for a personal reason.
Now imagine that chemical dependancy is in the form of a delusional egotist who could at any time just get the whim that all of one particular group or all of one certain people should die because it's what God wants. All the millions, all the billions, all the hundreds of thousands who are dependent as as a drug addict to this persons whims will do exactly as they are told in the name of God.
History attests to this Uni. ISIS attests to it right now. These people who blindly follow a book, or a man could randomly just decide God wants them to do something insane. It's far more likely than that drunk ever doing anything to anyone else.