Many of the people in these countries have lived pretty cruddy lives filled with war, poverty and other things that will just flat out make you mad. We have seen what people can do when tragedy strikes. (look at America pre and post 9/11) They have leaders who use them as cattle and there is violence all around. All it takes is someone fanning the flames and entire generations of terrorists are born!
What makes this worse though is the added aspect of belief. It is bad enough to be filled with hatred and anger, but when you have a sense of righteousness it is the final straw. (this is true of any religion, not just Islam) It is easy to convince yourself mass murder is ok when you know in your heart of hearts that God thinks what you are doing is great!
I think Salmon Rushdie said it best the other day on Bill Maher...there has been a fundamental shift in Islam and the violence and hatred, which used to be the extreme edge of the religion is now more mainstream than ever. what is worse is that even the people who distance themselves or dont support this fundamentalism also do nothing to try and stop it. It wasnt all that long ago that the Mid East was a pretty open place. There has been a serious shift.