I'm damned if I'm going to waste my only bullet on some random stranger when there's a psycho with a sniper rifle out there (assuming there's only 1 bullet, I'd check).
Sniper sounds scary until you realise that they've exchanged scope of vision for range, i.e. they can see far away, but only a small bit of far away. If I can't see him then if I dodge a bit he can't see me.
Yeah he'll shoot the guy, like I care. The key here is that while the other guy will be dead either way I have a choice:
a) He can be dead and I have an empty gun, or
b) He can be dead and I have a single shot.
I choose the situation that leaves me with at least a chance of survival.
Oh, plus if he kills the other guy then muzzle flash and the sound of the shot will give me some indication of which direction he's in (even with sound suppression a sniper rifle is pretty loud).
I realise that technically this puts me on the "evil" side of the equation, but it also puts me on the "armed" and "alive" side of the equation too, so I'm happy with my decision.