I think most people who hate the christian faith were people that were forced to observe the religion when they were young and as they grow up they just become bitter. I mean the reasons are different for everyone, for example they get introduced to people who already hate the religion and just bash it and bring up all the illogical/improbable things in the bible that the church tells people happened literally. Another big reason is if a person or their family member suffers a lot or has died when that person thought they shouldn't and then they're mad at God for 'letting that happen'. Some people don't like the contradicting morals in the bible, some people don't even have a problem with the bible but the church itself (like yours truly). Some churches become too prejudice or certain groups will start doing bad stuff in Christianity's name, and then when that happens the media of course has to capitalize on it giving the religion and the church bad PR.
It's all about the public face of the religion. If you do something in the name of your God or your beliefs that don't correspond with the religion itself or are unethical by today's standards, you're going to make yourself and your faith look bad. Then people concentrate not on the good the faith provides, but only the bad because that's the only thing that interests people, when the system fails, not when it succeeds. That in itself is almost more pathetic than the people doing weird or unethical things in the name of their god, but that's how our world works a lot of the time.
I wouldn't call myself an atheist, but I have a lot of friends that are and I know their reasons and I understand and often can relate to their reasons for not liking religion. Probably my main problem with religion is the constant pressure and insistence church people put on us sinners, as if we need reminding that time is short because I think most of us know and we get sick of being reminded of it. (and a lot of us try to escape the thought thru means we often regret)
I firmly believe that if a person is to have faith it must be a journey of self discovery for it to truly mean something, which is why I dislike the church so much. You have to want to do it, and it has to be for the right reasons otherwise following it is shallow and meaningless. The church tries to be comforting and encouraging (at least some of them do) but they don't realize that even when they're trying to be their nicest they sometimes can still scare people away. I also have friends who've found the right formula; who really believe in God in ways that are not shallow or two-faced and they truly are happier because of it. And I honestly hope someday I become a big enough person that I can do that too, because beneath all the doubt and politics and bad PR there is only the idea itself which is very powerful and truly comforting.