Because people can easily fall into religion and believe it more than believing in the grass or the sun. For a lot of people it becomes a way of life and a part of who you are. And when two religions disagree, it is easy for people to get mad at one another. Sometimes in groups, which can include a whole country, starting a war.
People want something to blame for there problems, some people have used religions the reason why there killing(even if that religion says not to) more have been created over the centuries. Also when evolution was created many people wanted to get rid of religion completely.
Because some people think their religion is better, people get mad and a war starts. or people believe there is something holy, like in the crusades, and think killing people will get them to their goal.