Harry listened, eyes wide behind his spectacles, feeling a bit sick. He'd seen what happened when you drank that. Remembered forcing it down Dumbledore's throat all-too-clearly. And he remembered the inferi in the lake. He was starting to think maybe he understood, and he wasn't sure he wanted to.
R.A.B. had turned his back on Voldemort, and he'd died. Or just bloody vanished, really. Harry didn't know what had happened, really. Sirius had just been guessing even, really. He hadn't known. But Regulus had helped Voldemort put the bloody thing there, and Kreacher was mad and sickly looking now because he'd swallowed that shite down. He'd probably been a nutter to begin with. . . but he hadn't even had a choice. He'd just had to follow orders and drink that. "I'm sorry," Harry muttered quietly, looking at Kreacher and then swallowing hard once.
"He brought you back, and he switched the lockets, didn't he?" Harry asked. "Your Master - Regulus - he brought you back, and he switched them out. What happened?" Someone had to have drunk the potion to get to it. That was probably what happened to Regulus. Maybe they'd gotten him when he was weak from that, and killed him. Or maybe. . .
Harry couldn't actually think of a much worse death than those things in the lake, really.