User blog comment:Alchimous/Theoretically speaking.../@comment-69.19.14.21-20121108014546

Considering that Guthix heavily intertwined with his partial creation, Gielinor, I suspect that attempting to kill any form he might take would, regardless of weapon used, be completely futile; Guthix would either not die, or would simply abandon that form and take a new form in a vastly different location (it is also possible that he can exist without a form at all). However, he would still be quite alive. If anything, attempting to kill him would only awaken him. Which would be bad.

Since Guthix will always exist as long as Gielinor does, I believe that the only way to 'kill' Guthix would be to destroy Gielinor itself. If so, then it partially explains why Guthix put the Edicts of Guthix into place.

Also, regarding the Godsword: The Godsword was created with the intention of killing Zamorak, meaning that the Godsword would be best used for destroying deities who have means of power similar to his. While the Godsword would be excellent for murdering powerful beings with localized power, it wouldn't do much of anything against a deity like Guthix.