Do you call them Christian because they call themselves Christian? Or because the government calls them Christian? Or because the media calls them Christian? Or because some friends call them Christian? Why?
Why do we call the ISIS as an Islamist organization? It is because they call themselves Muslim, attacks non-Muslims and force the non-Muslims to convert to Islam. The same case here. Organizations such as the NLFT and the NSCN kill Hindus because they don't want to convert to Christianity. And most of their funds come from Baptist churches in the Southern United States.
The Koran and the Hadiths are the backbone of Islam. These writings tell Muslims to be violent against people of other religions who will not convert, and against Muslims who don't practice their religion strongly enough -
http://www.thereligionofpeace.com/quran/023-violence.htm.
The Bible is the backbone of Christianity. The Bible tells nobody to be violent against people of other religions. The Bible DOES allow for self defense. Old Testament writings were for Old Testament Israel. They are not for Christians to follow as directives today. The word "Christian" is not mentioned once in the Old Testament.
So-called Christians who are aggressively violent against people of other religions are going against the Bible. When they go against the Bible, are they really Christian? If they are, they are extremely weak Christians.
Muslims who do not become violent against people who do not convert to Islam, and against Muslims who do not uphold their religion, are going against their religious writings. This makes ISIS one of the most correct Islamic groups around.