Why do people from different religions argue?

I want your point of view. Why do people who claim to be "holy" go about slandering others religions and treating people who do not believe in their ideals like lower life forms? I am a protestant christian and my god says to forgive others and to pity those who do not accept him into their lives and I try to follow that for he made all of us equal. From what I've seen and heard from the Qua-ran Allah has the same lessons so why is it that we cant get along? Best answer will go to the person who elaborates on their opinion and give an insightful answer.