A mix of owned and Killzone 2.
This urban word spread on gaming forums from the revealing of Killzone 2 at E3 2007, which despite many doubts and bashing, still received positive feedback. The word then became used by some fans to go against those that claimed the game will be a complete failure or went on to use it after a Helghast or other soldier got popped in the head, or humiliatingly killed.
(Helghast soldier gets annihilated.) "Killzowned."