The question should actually be: why do fundamentalists kill people who think different!
Because they define their own worthiness solely by what they consider unworthiness of others.
Its pathetic i no. But we all do this in one way or another. Nobody wants to feel worthless.
Most people try to increase their worth by actually doing something meaningful.
Some (the fundamentalists) overcompensate by doing what they believe not just meaningful but epic by following their gods/leaders will. They are too stupid or blind to see that they are just exploited.
Same goes for christians too btw.