You just said that Islam is decentralized. These are one of the decentralized factions. There are Muslims all over who want peace, but there are two problems with them. One problem is shown in this story, the other is that what they do never makes the news. Terrorists are news, people just living their lives are not. That is why so many people don't believe they exist.
There haven't been any meaningful religious wars in Christendom for centuries
And Islam is over 600 years younger than Christianity. In the middle ages, those of the then 1,500 year-old Buddhist religion were probably wondering why Christians were running around killing people so much.
Muslims have less of an excuse though. Today's technology should allow pretty much everyone to get the message that religious wars are not a good thing. Unfortunately, most of the terrorists get their news from only one source: their demagogue Mullahs.