ok just gotta throw this in there cause i think you have the wrong idea about religion in general. all of the suffering was caused by men who used religion as an excuse. if you actually look at the writings of pretty much every religion, at no point does it advocate genocide as a form of serving god. its evil people who use it as a basis for their crimes against humanity to make themselves feel righteous.
as far as why people believe in deities, i think the belief in a higher being is innate in many people. they believe in someone or something more powerful than themselves when they see how infinitely big and intricate the universe is and how infinitely small and insignificant they are.
just my opinion