What is radicalism in relious thought? It would seem that you almost have to be a radical in order to keep your faithful following you, and you have to be a really big voice to keep so many people involved with you without question. The best way to keep the followers following you, and not questioning what you are saying is to keep them alarmed with the consequences of what happens if you don't follow.
Whether you are trying to convince the faithful why they are following you in this world, or why they should follow you in the next world, there are many reasons why they need to be so up tempo. Listen, if you really ever read the Bible, or the Koran, you would be shocked at what they say about other religions, and what is going to happen to them. And if you ever read the two books in any detail, you would see that religion is really about yourself.
So if religion is about allowing God into your own heart, why do you need to be so harsh against others? Well, if you aren't harsh against others, you might lose followers. If you lose followers, you might not be able to do the work of God on Earth. If you can't do the work of God on Earth, you might not get into Heaven. If you can't get into Heaven what is the point of living? Obviously to keep the whole flock in order, you have to say some really big things.
So if you are going to keep people paying attention to you, and you are going to keep people following you, you have to be a radical. You have to preach that following other religions is going to send you to a lake of fire, and that those who have different faith aren't as good as you. There has to be one religion in the eyes of the leaders who want to press forward with their own agenda. While I doubt God cares what you believe, the leaders of each faith feel the need to make sure that you follow only the "right" one.
Also, it could just be that these leaders feel like they aren't being listened to, or are being oppressed. However, relgious faith is all about redemption, and that technically shouldn't be a reason to be so radical. Burning buildings, and killing those of other faiths aren't going to solve problems. During the plague, Christians killed Jews, and did it help? Do terrorists killing innocent civilians really help? Of course not, but they are brainwashed into thinking they are helping God rid evil.
There is so much radicalism in religious thought and practice because that message sells to people. People will see the fire, and the brimstone, and the consequences for not following God, and will fall into line. Most of it is out of fear, and ignorance, but what do we really know about what God wants? Until he tells us what he wants, we only have the interpretations of humans to rely on.