Suicidal Marine

In the Halo 3 level Floodgate, shortly after passing the area with a Flamethrower, there will be a Marine in the corner talking to himself about his squad's infection by the Flood, and how he just "did them a favor".

After this, he says something about how he should do himself a favor too, and points his gun at his temple, seemingly about to kill himself, although he alternates between this and putting his gun down and shaking. Eventually the Marine will stand up in a combat ready stance, but will not accept other weapons. You can make the Marine stand by throwing a grenade near him.

Trivia

 * The Suicidal Marine is distinctly similar to the Halo: Combat Evolved's AWOL Marine, and indicates that Flood attacks can have profound psychological effects.
 * Interestingly, the Marine may occasionally be seen without his M6G, raising his hand to his head as if he had it in his hand. This is probably the result of a bug in the game.
 * This "suicidal" pose by the Marine is shared between several Voi Workers in The Storm.
 * If you look closely, the dead Marines lying around the room show no signs of Flood infection. It may be possible the Marine just lost it, and subsequently murdered his comrades in his crazed fear. On the other hand, it is also possible that he destroyed the visible signs of Flood infection.
 * If you hold the camera in this room in theater mode, he will start to get up after a short wait, then simply vanish.
 * The Suicidal Marine has unusually low health, and dies after only a couple of bullets if you shoot him, indicating he may be wounded, have damaged armor, or simply have no will to live.
 * If you go to Theater mode and fly to the Marine's location, you'll see the dead bodies, but not the Marine.