Author's Note: This isn't my usual cup of tea, but it's for AllIKnowIsADoorIntoTheDark, and she promised to love whoever wrote a dark or angsty Regulus/Peter fic. But I think I ship these two now.
Regulus squirmed, adjusting his gaze to a spot just over his cousin's shoulder. He wished Bellatrix would just shut up and get the hell out.

The woman seemed to pick up on his discomfort. A smile twisted her lips, and Bellatrix dropped into a chair, making herself at home. "As I was saying," she continued, grabbing an apple from the table, taking a bite and spitting it onto the floor with a look of disgust. "We have a bit of a situation."
With a sigh, Regulus shifted his weight to his left leg and frowned. "A situation?" he repeated, his bored tone laced with a touch of uncertainty and worry. "What sort of situation?"
"Loyalty among the others," Bellatrix said, sliding her finger over the thin layer of dust on the table before muttering something about useless elves. "They need to be tested."
Regulus couldn't fight a shudder. "Who?"

"That Pettigrew," she spat, eyes narrowing. A moment later, a cold, amused smile played at her lips. "And you."
Regulus felt his heart stop. If Bellatrix believed he was disloyal, nothing would stop her from killing him. As a precaution, his hand dipped into his pocket, fingers wrapping around his wand. "And why is our loyalty in question?" he demanded sharply, steadying himself.
Smirking, the woman climbed to her feet, circling her cousin like a bird would its prey. Her fingertips grazed his cheek on her final circuit. "I know who his friends are. Order members following that old fool blindly. And I know your brother, remember? The traitor."

"I'm nothing like Sirius!" Regulus snapped, growing angry.

"Then prove it," Bellatrix purred, leaning in to whisper in his ear.
Regulus went pale, his eyes widening at what was being demanded of him. "I...I understand," he sighed miserably, hanging his head in defeat.

OoOoO

Peter was clearly afraid. His beady eyes flickered rapidly from spot to spot as he wrung his hands together. Of course, no one had explained to him exactly why he had been brought to the Grimmauld Place cellar. "R-Regulus," he gasped as his fellow Death Eater joined him.
Regulus tried to appear apathetic, but it was difficult. This wasn't like him. While it was true he was nothing like his brother, the man was even less like his twisted cousin. "There are two ways this can be done," he said coolly, caressing his wand.

"This?"

"The easy way, with you submitting like a loyal servant of the Dark Lord," the younger man continued, as though the other hadn't spoken. "Or the hard way, with you resisting like the spineless whelp that you are."

"I-I don't understand. What am I-?"
"A few of us have come to question your loyalty, Pettigrew. We're afraid the traitor may betray us as well."
"Never!" Peter shrieked frantically, shaking his head. "I wouldn't! Please believe me!"
Regulus lifted a brow, watching the man pace a short distance before stopping again.

"I'm faithful!" he insisted, gazing pleadingly at Regulus.
"Then prove it," Regulus said, echoing his cousin's challenge. His wand lifted menacingly. "If I have to restrain you, this will be much worse."
Peter swallowed dryly, nodding slowly. Even in the dim light, his tremble was visible. "I won't struggle," he whispered weakly.
"Good," Regulus muttered. "Let this serve as a reminder that we are not to be crossed, and if you try, things will be worse. Crucio!"

The curse knocked the older man off balance. Peter writhed on the dusty floor, shrieking.
"Crucio!" Regulus could feel his stomach twisting, stomach acid burning his throat, disgusted at his own monstrosity. "Crucio!"
The screams grew louder. Peter's limbs contorted at grotesque angles, his body convulsing. Regulus paused, letting the latest curse work its way through.
"Please..."

Regulus hesitated, his hand shaking. "Crucio!" he repeated, eyes closing as though that could protect him from the pained howls that filled the air once again. Only when the room grew silent did Regulus dare open his eyes again, moving his wand to his pocket.
Bellatrix would have continued. Hell, most Of the other Death Eaters would have driven Peter past the point of breaking. But Regulus wasn't like them. He wasn't like anyone, really.
Peter twitched on the floor, his face pale and dotted with beads if sweat. A pitiful whimper escaped his lips as he tried to sit up, only to collapse with groan. He was so broken, so utterly helpless, and it was Regulus' fault.
The younger man knelt beside Peter on the floor. An apology hung on the tip of his tongue, but he swallowed it down, unable to let it fall. How exactly did one apologize for that level of wickedness? Regulus doubted it was possible.
Instead, he helped Peter into a sitting position, stroking his light hair. Peter curled against him, muttering weakly about promised loyalty.

"I know," Regulus whispered. "I know."

He pulled back, gazing at Peter for just a moment before leaning in again, pressing their lips together. The elder gave a startled jerk before slowly relaxing into it.
Regulus wasn't sure what had possessed him. Maybe he was simply desperate to offer some semblance of comfort after the damage he'd inflicted. It was supposed to mean nothing, and yet, Regulus found himself melting into the kiss, working his lips needily against Peter's.
Peter was the one to break the kiss. He peered up at Regulus in confusion. "I-"

"I'm sorry," Regulus mumbled, unsure if he was apologizing for the curses or for the kiss.

"Don't be," the other whispered, shaking his head. He looked nervously around the room once more. "Can I...Can I go?"
Regulus nodded, watching him climb shakily to his feet. As Peter's figure retreated, Regulus couldn't help but wonder if he'd made a mistake in joining the Dark Lord.
He wasn't cold like the others. Even if he believed in the sanctity of blood purity, Regulus was hardly manic about it. Still, there was family pressure and the unshakable fear of not doing what was expected of him.
Sighing, Regulus pulled himself up off the floor, dusting his trousers. Once, he'd been so sure he was on the right path. Now, his faith was shaken.

OoOoO

Peter became a surprisingly frequent guest. Though Regulus assured a constantly suspicious Bellatrix that he was just keeping an eye on Peter, their meetings grew into something more. There little kisses and exchanged secrets, late nights spent watching the flames in the fireplace die down to embers.
Regulus trusted him more than he'd ever trusted anyone else. Peter was a traitor. They both knew it. He would understand Regulus' newly found doubts. But Regulus couldn't bring himself to voice them.
"Is it easy?" he whispered, running his hand over Peter's arm. "Betraying the ones you care about?"

"No," Peter mumbled against his lover's chest. He looked up at Regulus with a small frown. "Why?"
The younger of the two shrugged, trying to come off as casual. "It just seems like it would be," he said quickly.
Peter pulled away, his frown deepening. "You just don't understand, do you? You cross them, whatever your intentions are, and you still have to see them, carrying the secret that you plan to destroy them. You live a lie, and the guilt eats away at you," he said sharply.
Regulus flinched at Peter's tone, holding his hands up defensively. "Okay, okay! Please... Let's not fight. I was only asking."

With a sigh, Peter curled close again, nodding. "Sorry," he murmured.
They had questioned Peter's loyalty. Of course, he had been the obvious one to worry about. But maybe, just maybe, Regulus was the only they should have doubted all along.