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 bears a striking resemblance of the Crazed Marine in Halo: Combat Evolved that shoots you and says, "Stay back! You're not turning me into one of those things! I'll blow your brains out, get away from me! (screams) Don't touch me you freak, I won't be like them, no I'll die first! Play dead, that's what I did, play dead, they took the live ones, oh god I don't want to be like them. (screams more) JUST LEAVE ME ALONE! (keeps shooting you)"
 * 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.
 * 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 just "disappeared" 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. Also, the skin color changes. Occasionally, he will spawn with a radio on his back.
 * 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 by your first hit).
 * If a co-op player, such as the Arbiter, hits him he will not die or be injured in any way.
 * The Crazed Marine in Halo: Combat Evolved, and the Suicidal Marine are voiced by the same person.