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.