I know that wikis are often inaccurate but I just had a good laugh at the entry for Matt Decker on Memory Alpha. Whoever edited the article referred to the planet killer as a "creature" and that it "died" when Kirk took the Constellation down it's maw.
http://memory-alpha.org/wiki/Matt_Decker
http://memory-alpha.org/wiki/Matt_Decker
Decker was killed in the attempt. While the attack failed to cause significant damage - the Planet Killer had lost a small amount of power - it did demonstrate to the Enterprise crew how to disable the creature. Kirk drove the Constellation into the maw of the Planet Killer, which died when it was unable to handle the energy released by the Constellation's exploding impulse engines.