Why do Christians and Islam fight to each other although they believe in the same god?

Why do Christians and Islam fight to each other although they believe in the same god?

Generally, religions are the instruments for mankind to access the happiness in their life. The core of each religion consist of good purposes to bring happiness to participants as a whole. The true happiness of someone should intimidate those of others. However, I have seen many violent events that has resulted from the conflict between christianity and islam. As a viewpoint of a man who is neither christians nor islam, I am curious why christians and islam have resisted each other and led to several wars and violences although they have the same god and share apart of the belief. 

A root of conflict between christianity and islam is “Crusades War”, one of the greatest war in the world’s history. In 1095, the christians desired to get back the holy city, Jerusalem, from muslims who also believe in the holy city. There were countless deaths related to the wars. In my opinion, the actual reasons behind the religions that created conflict between christianity and islam are political power. In fact, because religionists want to expand their own political power over the other, they just find some excuses related to religions to prove their right to destroy their enemy. 

Personally, I don’t agree with any religion leader who claims that killing other people who have different beliefs is right. I believe that the core of every religion is pure and good. No religion accepts that killing other people is not guilty. In some extend, people might forget why they believe in a particular religion and how that religion is supposed to influence their life. Is it not the happiness that people need?