My theory is that he was killed to allow the game's lore to grow. Lore was meant to be one of the important things about rs3, with world events and all these cool stuff.
Before his death there were no gods, and besides the 3 or 4 incomplete quest storylines (more like 50 but yeah) there was little room to create new stories. With gods present a larger variety of stories may be created. It just adds a new dimension in the lore and increases its complexity.
In the end of diablo 2 a similar event happened, the worldstone was shattered and as a result demons and angels can now manifest in the world. Diablo 3 was not that amazing despite that though lol.
When creating games, you can often add a weird restriction to prevent your work from being chaotic, and that can be justified through lore, either straight away or later on. When needed in the future, that restriction may be removed and the appropriate lore will explain what happened.
Hi
23-Oct-2014 16:09:40