Suicidal Marine

In the Halo 3 level Floodgate, shortly after you can get the 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 cannot be pushed, will not respond to proximity and will not accept other weapons.

Trivia

 * This marine is a striking resemblance of the Crazed Marine in Halo: Combat Evolved that shoots you and says, "Get away from me!! You're not turning me into one of those things! Oh god, oh god! Jenkins, Johnson, Bisenti...they were all infected!! Just get away from me!!! Oh god I played dead...Yeah that's what I did, play dead. Just stay away from me! (mumbles to himself)"
 * Interestingly, the marine will sometimes not be armed with any weapon at all, raising his hand to his head as if he had a gun, even though he has none.
 * If you look closely, the dead marines laying 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.