An "in group" literally requires an "out group" to exist as an "in group", it seems pretty basic that opposites imply one another. It's only when people don't realize this that they want to eliminate completely the group they consider to be the enemy. On what grounds will you call yourself righteous when you've killed everybody not righteous? Well, you take a page from our government and invent a new "out group" for your "in group" to kill. Having some type of opposition is very much an essential part of politics, else you find yourself in the position we are: tyranny.