He's close to being pure evil, but even then he has love for his mom, Jaime, Tywinn, brother and sister. He also genuinely loved and looked up to Robert.
I'd say the Mountain is pure evil. He's pretty much a killing, raping, and torturing machine, at least in the books. We never find out too much about him, but it's always horrible news for anyone around him and he takes great pleasure in it.
Another pure evil character might be [spoiler]Ramsay Bolton.[/spoiler]