Only a god/divine creature can kill another god.
Examples:
Ophiotaurus- If it was killed, and its entrails were sacrificed in a fire, the subject to do the ritual would be bestowed with the power to overthrow the Olympian gods. Some say it's just them disappearing, some say it's them being imprisoned, some say it's them being actually killed.
Fenris (Fenrisulfr)- During Ragnarok, Fenris is fated (along with Jormungandr, the Frost Giants, and several other monsters and groups of monsters) to kill off the gods in a most brutal fashion.
“…Judge not what a man has done, but judge what he could have done if he was a different bloke altogether. For art thou a leper? And a leper can changeth his spots…” --Rudy Wade, Misfits (Series 4, Episode 8)