I love how at first, Sozin legitimately came to help his friend fight a volcano. It was only when he realized Roku was the only thing stopping him from enacting his plan, that he chose to leave him there. It's as Aang said, anyone's capable of great good and great evil.
Also, Roku's earthbending teacher, Sud, is the earthbender you see in the opening. He was originally supposed to be Aang's teacher, but then they decided on Toph and it ended up being one of their best decisions.