Many people ask the question of "Where does Islamic radicalism come from?", which some people who hate Muslims, such as Pamela Geller, will try to explain by saying that Islam is Evil. However, they bypass the real problem. It is not Islam that is evil, however there are some people who may be evil who do things in the name of Islam. You could say that no other group has done more harm and damage to Islam as much as the Wahabist Islamist Neo-Salafis from Saudi Arabia which you can see speaking of how to treat Christians, which is against all Islamic norms of tolerance and understanding.
Next time your in a Mosque, check to see if the books are published by groups which support the view of the guy in this video. If they are, you know that you are not dealing with mainstream Sunni, Shia or Sufi Muslims.