Disclaimer: I'm not J.K. Rowling why would she be writing fanfiction when she writes the canon?
I wrote this before the seventh book where we find out what actually happens the Regulus. So let's call this an AU where he lives. I may write more to this but don't count on it.
He was facing a Death Eater with a wand to his throat and all he could do was laugh. Regulus did not think he would laugh in the face of his death. That was always something he thought Sirius would do, die laughing. He just found it funny which Death Eater had found him.
"What's taking you so long, Serveus? If Bella would have found me first, I would have been long gone." Regulus asked with a smirk.
"Why did you betray the Dark Lord, Regulus?" Snape asked, his wand digging deeper into Regulus' neck.
Regulus laughed again and felt sparks burn him from the wand but he did not pull away, he instead embraced the pain. "Because he is not even a pureblood! He lied about that then I found out what else he has been lying about. I uncovered his dirty secrets even the one of his immortality. He is quite mortal, if you get rid of a few minor details."
Regulus stretched his hand out to show a locket fall from it, his hand still holding the chain. The heavyset locket had a serpentine S on the front but it looked worse for wear with a crack in the middle of the S. "This was one of those minor details." Regulus laughed again and Snape's wand faltered, "Why won't you kill me?" he asked angrily, "Lost your nerve?"Snape glared at his best friend and raised his wand again. "Don't tempt me, Black." he snarled.
"Your orders are to kill me. Why won't you?"
"Because you were my friend once and I want to know you aren't..." Serveus faltered.
"Under the Imperius? Not a chance, this is all me. I took a piece of his soul and destroyed it. And I'd do it again! You should get out while you still can, Serveus. Oh, wait, I've forgotten, once you get marked there's no way out!" Regulus yelled, holding up his marked arm.
"...there is a way." Snape muttered.
Regulus looked up at Snape in surprise. "What way?"
"I'll fake your death and hide you from the Dark Lord." Snape stated, ignoring the specifics of the question.
"Serveus... He'll kill you for this." Regulus gaped.
"I am confident that he will never find out from me." Snape stated, lowering his wand.
Regulus rubbed his neck, careful of the burn marks from Snape's wand. "And how is that? The Dark Lord is a master Legilimens."
"Ah yes, but I am a master Occlumens, better then he is a Legilimens." Snape countered, "Hurry, we haven't much time before the Lestranges arrives and I assure you they will not be as lenient as I."
Regulus followed Snape as he led him to muggle London. "Serveus, thank you..." he said sincerely but Snape continued as if he had not heard a word Regulus had said. He was already preparing his mind for this lie he would have to life with the rest of his life and a secret he would have to keep until the grave.