Catholics and Protestants are both Christians, Catholicism is pretty unified in doctrine, and Protestants are a large group of very diverse denominations of Christianity. When I was forced to go to church, my pastor would often joke about Baptists forming new denominations over differing opinions on drape colors. Outside of those two groups, there are tons of other flavors of Christianity. Off the top of my head you've got Mormons, Greek Orthodox, Eastern Orthodox, jehovas witnesses, and Oriental Orthodox
Just as "extremists" from other religion claiming they belong to it. We're just less accustomed to seeing people claiming being chrisitans doing it. They obviously aren't but as time will go on more and more christians will get criticized for being christians because "look what your fellow christians do!" Rings any bell? "But if Christians behave that way it's because it's rooted in their religion!". And maybe it is... Who knows, but what these people do seem very non-christian.