Quite simply they feel their hate is justified while the hatred of others is not. They are unable to see themselves as others do. Or they don't care about what others think.
Pearl hit the nail on the head.. I am an observer of people.. a people-watcher.. And I have observed this so many times.. And I've seen it countless times.. people who regularly engage in unpalatable language... then seem to be the first to criticise others for the same thing.. I am puzzled by it.. but I can only think that they seem genuinely unable to see and identify that when they do it, it's the same thing.. Humans are a really peculiar bunch.