Christian should be viewed as a sort of umbrella term the same way Pagan is. Catholics, Lutherans, Anglicans, Episcopalians and Baptists, even Mormons are all Christian.
As to why the various denominations wage a verbal (and sometime more than verbal) war against each other, it has to do with the nature of a monotheistic, exclusivist faith. Each denomination believes that theirs is the only proper way to interpret the bible, to relate ot Jesus or to understand the relationship between man and their God. If their's is the only "true" way, then the others must be wrong, and since the others refuse to admit their error and convert to the "one true faith," they must be deliberately wicked.
It seems pretty damned stupid to me, to be honest. Look at all the blood shed in the name of the "prince of peace" between those who all claim to follow him. And people wonder why I left Christianity a long time ago and have never looked back!?