I will ignore the logical contradiction resulting from your wording, because there's a more important, less pedantic issue.

While you may call the person "NOT a Christian" because he doesn't follow all the morals of Christianity, he still calls himself "Christian". There have been many religious holy wars in the past, violence done for the sake of religion, even if you might consider them to be against the religion's values.

I do not blame the religion, anyway. No matter what your belief, if you take up a tribalistic, us-versus-them stance with anything, things resembling holy wars can occur. Religion can result in this attitude, but so can patriotism and many other things which are often praised.

While such causes may unite people, they only unite some people, and dehumanize everyone else.

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A very familiar and common example of this is gangs. People group up with the united cause of protecting their turf against "the others"... Pointless violence ensues. United causes strip reason and morality from people.