Whats about Stalin? Whats about americans and the indians? Whats about christian persecution by the islam? There are so many bad people and groups out there in the history of the human being... you cannot say this one or this one is the worst.

back to your question, why most people say muslims are terrorists. the majority of terrorist attacks is done by radical islamistic members. Sure, its wrong to say that all muslims are terrorists.... but i've never heard that anybody said that all muslims are terrorists. religions in general are bad in my opinion.
but the mindset of the radical islam is stone age... a woman is worth nothing. homosexuals were treated really bad and so on...