In Runescape, you kill Gielinor; a human-borne god, woven by a madman from the collective souls of the Planet's various underworlds (Death is the first person to die on the planet, also called Gielinor). it's basically a mindless puppet that's made of corpses, being kept alive by an arcane device that destroys and siphons living souls into itself.
though in Runescape, the traditional gods tend to be mortal-borne or just ascended in power by external means. the closest to a more omnipotent god would be Mah, an Elder God. though you only endure her Nightmares-given-life, after that quest, it's said that she's clawing through the multiverse to get some sort of primal revenge/hunger.
Other than that, you also passively kill some lesser gods in events, and partially during world events. Bandos and Tuska are dead though not directly by us. Gods in the Runescape universe are just empowered mortals who tends to have the ability to freely teleport, change size, fire beams and possibly shapeshift. whilst the Elder Gods themselves are nearly incomprehensible and create/destroy matter, and are pretty unkillable.
And Mah? the other elder gods thought she was stillborn, and she's been passively sucking an already dry planet...drier, a planet that's the only remains from the old universe before the time where the game's set. when the other elder gods wake up on the most perfect planet they produced, then they'll drain the energy from the entire universe, leaving that planet as a husk for the next universe and possibly the elder gods that hatches there.