[i]Perhaps he is a reincarnation of Gilgamesh? [/i] It is rather intriguing that he had survived, and has perplexed me since the start of the Halo franchise. Sure, they have come up with some elaborate expressions of answers to satisfy the majority, however once playing the Legendary ending, there can be no question on his destruction. The true answer however, is probably because he was a fun character which was reprogrammed in the sequel for good measure.