Like fear, pain was something a Space Marine might feel, but never bow to. Pain was the Emperor's way of reminding him there was work to do. An Astarte's duty overrode pain. It even overrode death.
That thought sent a rush of hatred and rage through him, and he reached down to his belt, pulling out a green vial with a long injector needle. Carrizales popped the cap off of it and jammed the needle into his thigh, hissing at the pain as the tip punctured his skin.
The pain still bit at him for a few seconds as he listened to his men fight and die all around him, slogging forward to be cut down. Then, the noise of battle began to dull, drawing back. Carrizales looked up at his enemies, hearing a deep, rapid drumbeat in his ears, overriding all other sounds. He could acutely smell the blood and feces and cordite in the air, even as the sensations from his skin slid away.''
The drumbeat grew faster, and he rose, no longer feeling pain, no longer feeling anything except his heightened sense of smell, like a hunting animal. He thought he could smell their fear, and could see a blur, indistinct but locked in his mind. Hatred began to grow within him, flaring up as he stepped through the water. Numbly, Carrizales raised his rifle.
Not every kid learns it the same way. Some learn it so good that pain loses all meaning. It just doesn't register. Prison guards call guys like that "anesthetics." When they go, they go. Clubs bounce off their heads; they wear mace like it was a coat of sweat; they pull stun-gun wires out of their bodies and strangle you with them.
"You can't hurt them. It takes death to stop their pain."
"My body senses injuries. The data could be called pain."
— The T-800, Terminator 2: Judgment Day
"Didn't they give you anything for the pain?!"
"I think we nee to step down your steroids a notch, Corporal."
— Nick and Dr. Bunnigus, Schlock Mercenary
"What, my hand? No, it's fine. A blaster shot just grazed it. No, no, it doesn't hurt. I don't let it."
"... By the way, you might see my arm lying around somewhere. If you could pick it up, that'd be great."
— Dhomochevsky, Blame!
"But the key difference between you and me? My body can't feel pain!"
— Nemesis Prime, Transformers Prime