- last post: 01.01.0001 12:00 AM PDT
You can't kill him because he is needed in the game, simple as that.
He knew the ship was going to explode, so he just teleported to a safe distance. Then he just flew away. Coming up with a reason he is nigh indestructable is pure speculation at this point. There is a monitor on each installation. The one for Delta Halo was Penitent Tangent 2401. Guity Spark 343's number is 7 cubed. 2401 is 4 to the fourth. I don't know what that means.
Johnson supposedly had an encounter with some plasma grenades or something that made him unsuitable or immune to being infected by the Flood. He, and some others as well, were not on the first Halo when it was destroyed, contrary to the special ending on Legendary (which Bungie said was just a bonus, and didn't actually happen) and what Cortana said as you were leaving.