Could be a multitude of reasons buddy. It's hard to know without knowing the person. Most people I know that "hate" Rangers would probably say they hate the fans, I only really hear this from knuckle draggers themselves, the type to use sectarian language whilst trying to pontificate about how bigoted a certain set of fans are.
I know a few celtc fans that don't like Rangers football club for obvious reasons that don't need to be said.