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. If you are lucky, you can push him to the area with the descending Elites, where you can push him off the cliff and let him get infected. When the Marine is infected, he will fight on your side and will assist in combating the Flood. If extremely lucky, the now infected Marine will spawn with a pistol that has unlimited ammo.

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. On the other hand, it is also possible that he destroyed the visible signs of Flood infection.
 * 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. Also, the skin color changes.
 * Depending on the game difficulty, you could smack some sense back into him by meleeing him causing him to either follow you or just stand where he is(if he doesn't die).
 * If a co-op player, such as the Arbiter, hits him he will not die.
 * The Crazed Marine in Halo: Combat Evolved, and the Suicidal Marine are voiced by the same person.