The world of Gelinor is polytheistic, therefore think of the entire world as having one religion rather than a seperate on for each god. All people that believe in one god of Gelinor believe in all of the gods, they just do not support them all. That is a subtle but extremely important difference. Therefore, a Sardominist would not claim that Zaros does not exist, instead he would claim that the worship of Zaros is evil and Zaros is inferior. (This is all based on ancient Greek and Roman theology, and modern Hinduism) As for the Nature of Guthix, Guthix is above all a god of balance and nuetrality. If you look at the natural world, death is a part of life. The over growth of plants and animalscan harm the world just as much as the death of them. It is true that Guthix might have to actively oppose one of his brothers but in doing so he would not ally himself with the other. I do not know enough about the other gods to make a statement about them.