"I told you I'd come get you if I got out. I promised you. And then I didn't. That's abandonement, Reg. I should've-- died rather than leave you there."

Is dying a little, still, knowing Regulus is still there. Knowing he hadn't figured out a way to do it without bringing their parents down on them both-- on Regulus most of all.

A gentle bump of his forehead against his brother's, "We'll-- we'll find a way. Remus 's smart. A lot-- a lot smarter than me. We'll figure it out, no matter what. I promise, and I mean it this time."

His voice trembles only slightly around Remus' name. Hopefully Regulus just attributes it to the moment.