DOUBT is the very foundation of fanatacism. At the core of a fanatic is a person who hasn't connected all the dots. A fanatic doesn't have faith. They are affraid of making the leap of faith. So they stand at the edge of the abyss and just jump up and down - never getting to the other side.
Instead of facing their own doubts about what they "really" know they pick on others for having doubts.
Fanatics have tough lives and eventually either surrender to their doubts or kill themselves because they believe infidels deserve death. Sometimes they kill others in the process. We should never underestimate the dangers of dealing with fanatics. If you find you are dealing with a fanatic its best to leave them alone - for your own safety.