In a battle against relics from an alien war, hostile to all life, the human race finds justification at last for its love of weapons: When the hour struck and our enemy came without warning, you were ready with swarming battlefleets. You were dispersed and dug in on scores of planets, and heavily armed. Because you were, some of you and some of us are now alive.
The unarmed pacifist with a terminal illness, sent up to negotiate with an alien planet-cracker. He agrees to provide a piece of human flesh (his own) with which the Berserker can develop a bioweapon, and to carry said bioweapon back down to the planet and infect it. What he gives it is a piece of his metastatic cancer.
The Earth commander who takes refuge in an impenetrable capsule deep within the gravity well of a black hole, using it to stay (relatively) immortal until descendants of the humans he fought alongside can find a way to rescue him. Whenever supposed rescuers arrive, he tests them by telling them to "look around at the scenery for a while". Only the genuine humans are freaked out by it the way he was when he first saw it.