That's what im talking about tho. How did he become loyal to the lannisters in the first place? I knew he was one of their knights, but from the picture they paint of him since childhood, seems like it would take more than knighthood to buy that type of loyalty from him. he's been doing their dirt for decades.
He's not loyal. Think of him as a sellsword. The Mountain, as crazy violent as he is, understands that the Lannisters, specifically Tywin offer him a lot of clout and autonomy. By serving the Lannisters he gets to do what he likes. Any other houses would probably and at his activities and send a bunch of dudes to kill him off.