Ive served a total of 27 months in Iraq, working shoulder to shoulder with Iraqi Army and Police and civilians. I can tell you from 1st hand knowledge that the terrorists do not represent the Islamic or Muslim majority. The problem is that in a lot of the regions of Muslim countries the population cannot read or write, so all they have to go off of is the extremist cleric that tells them that when the Koran says "convert everyone to Islam" what it really means is that those that wont must die, they don't know any better. Then that spreads. Muslims from America and other places see us killing Muslims and it makes them want to help them.
Think about it like this, If a Christian Army invaded Israel and Russia came to its defense and started wrecking shop there would be a lot of Christians that would join the "Christian Cause" even if it was wrong.