The entire "allowed himself to die" seems a bit strange actually, Guthix in TWW stated that he could have stopped Sliske at any time prior to his being attacked, but that implies that once he was struck by the staff he was a dead man, no ifs no buts. So he could have stopped him before the attack but not during or after.
Zaros on the other hand withstood such an attack and continued to fight on. In fact, were it not for him "tripping" - will cover this later - Zamorak would have been the one headed to an early grave as all the lore has so far has indicated. It was the "trip" that led to his - Zaros- loss, not the act of the Staff being driven through his back.
The part where the Staff is called the "Siphon" actually makes this make sense. Simply impaling someone will only create a puncture wound, nothing more. In a divine being, this is hardly damaging as they have no actual body that relies on functional organs that would be damaged as mortal beings would. What the Siphon would need, is a receiving end. That is, an in, and an out. So Zamorak being impaled on the other end provided this as Zaros then had his power "siphoned" out, and into Zamorak upon which Zaros was greatly weakened.
Guthix's death, however, throws this into the wind as he was struck by a magical attack, even if it was through the Staff it is still, a magical attack, even if from a Mahjaraat, it's still, simply a magical attack that a god should be immune to. There was no receiving end as in the case of Zaros, so the idea of Guthix dying is actually very strange and not fully explained.
But either way, so far it seems like a fraction of residual energy from Guthix is enough to overpower an Elder Artifact - this is... not consistent with what the lore says about Elder Artifacts being immeasurably powerful beyond that of any god as any god that possesses them is augmented further without exception. This is only possible if the artifact is stronger than the god in question.
16-Jun-2016 16:22:53