I actually think his tale is somewhat tragic. He apparently stumbled into the valley at the time of the civilisation forming. He had the pyramids built - especially one for himself and was able to recharge his life that way - basically never getting older. I don't think he was necessarily evil, like Vorbis in Small Gods. He was just incredibly focused on keeping the civilisation going. The people weren't important to him so much, except as a means to build bigger and better pyramids. He was basically the ruler, even if he probably wouldn't have acknowledged the fact.
The true tragedy is that he gets blasted back in time with only a limited memory, to the point in time when had first arrived at the valley. He wanted to achieve a form of immortality, but what he got was an infinity of living his life over and over.
The name Dios suggests the Spanish word for "god" and I'm certain that was not a coincidence when Terry was writing the book.