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 will not accept other weapons. You can make the Marine stand by throwing a grenade near him.

Trivia

 * If you go forward and leave this room for a little bit and come back you will find him dead or not even there. A pistol will lay on the ground. If you pick it up it will be missing one bullet. The marine probably shot himself, although it is not possible to hear a gunshot.
 * This Marine bears a striking resemblance of the AWOL Marine in Halo: Combat Evolved that shoots you and says, "Stay back! You're not turning me into one of those things!" and other things. Also, the Crazed Marine in Halo: Combat Evolved, and the Suicidal Marine are voiced by the same person.
 * 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 no weapon. This is probably the result of a bug or glitch in the game. It could also be that the Marine is distraught and doesn't realize he's unarmed.
 * 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. Another possibility is that the infection may just "disappear". It happens to bodies that aren't completely infected, after awhile, the infection just disappears off the body.
 * The Marine's helmet changes every time you play the level and sometimes he'll be wearing a specialist helmet, basic Marine helmet, or no helmet at all. Occasionally, he will spawn with a radio on his back.
 * If you look at him in theater mode, when the cut scene with the Covenant Carrier, Shadow of Intent starts he will get up and head for the door.