Regulus knew he wasn't going to get far. You could only avoid the Dark Lord so long before he killed you, or sent someone to kill you. Regulus knew perfectly well it would be the latter for him. The Dark Lord didn't hold him in high enough esteem to come after him in person.
He knew it wouldn't be long. He had been on the run for weeks, he supposed (though he had stopped counting; for all he knew his reckoning was totally off), going for increasingly longer periods without food or sleep until he had forgone them altogether, depriving himself for the sake of survival.
He knew in the end it wouldn't matter. He wasn't the least bit surprised when a Death Eater burst into his pitiful hiding place—an abandoned little Muggle house, falling into decay. He was disarmed before he could even properly react.
"This is the end, Black," came the Death Eater's voice. "You have defied the Dark Lord, and you will pay the price for it."
Regulus started a bit. "That voice…" he said, "I know that voice."
The Death Eater made a sound in his throat that was no doubt intended to come off as indifferent, but even a troll could've sensed the nervousness behind the bravado.
"…You were one of my brother's friends in school."
"So what if I was?" came the sharp rejoinder.
"Yes… Peter Pettigrew. …They called you Wormtail, right?
"Shut up!"
"Wormtail… how ironically fitting."
"That's enough out of you, Black!"
"So you've run off and betrayed you friends…"
"I need no friends with the Dark Lord's protection!" Pettigrew spat.
Regulus gave him a crooked smile. "Keep telling yourself that, Wormtail."
"I told you to shut up!"
"I take it you've only recently joined… I think would have remembered you."
"Your death will prove to the Dark Lord that my devotion to him is sincere."
Regulus laughed hollowly. "He holds me in less regard than I even imagined."
Pettigrew brandished his wand and bellowed, "CRUCIO!"
Regulus screamed until Pettigrew lifted the curse. "I… seem to have touched a nerve," he said breathlessly. He watched Pettigrew's hand twitch a little. "Look at you… you're afraid. You don't want to do this."
"That's enough out you already, traitor!" spat Pettigrew.
"Traitor?" Regulus laughed. "Is the pot calling the kettle black?"
"You—"
"I know what I've done, and I know the price I'm to pay. The same you'll pay someday."
The Death Eater's voice was sharp. "What?"
"Death. It's the fate of all traitors, Wormtail. I'm not exempt from it, and neither are you."
Pettigrew made a contemptible noise.
Regulus continued, "We're alike, you and I, but with one difference. I'm not afraid. You… you're afraid of everything. You're even afraid to kill me—why else have you prolonged this so much? And you're afraid of death. But I promise you: one day you'll get it."
"AVADA KEDAVRA!"
Regulus didn't have time to even blink before the jet of green light struck him in the chest. Peter Pettigrew stood there in attempt to steady his ragged breathing, stowing his wand away with a violently shaking hand. Finally he calmed down. What did Black know? Black was the one who had been reduced to a corpse, not him. His words meant nothing.
Pettigrew spat on the body, turned, and strode out of the house. He blasted the Dark Mark above the building and Disapparated.