"What in hell are you DOING?" Harry had called things off with Ginny. He had good reasons. It was the sensible thing to do, and left her less likely to be hurt, in the long run. And he had things to do and worrying over Ginny and how nice her lips were or how she smelled and how much he wished that kiss had gone better - it wasn't helping anything.
None of that meant he wanted to walk in on her snogging someone else. And it hadn't looked like a quick snog, either. And Capper wasn't Corner, or Thomas, or any of the other blokes he'd KNOWN he should be jealous about liking Ginny, even if hadn't wanted to be. Capper wasn't even a bloke.
Harry normally likely would have just stammered and darted away to nurse his wounds and be really bloody confused in private. Or maybe gone to talk to Hermione. But he had the locket tucked in his pocket, and negative emotions and fury were a lot easier to access than anything else. His cheeks flushed red and his jaw went tight, staring at them still. "So you just bring anyone up here to snog now?" he asked Ginny flatly. "Did you run through all the blokes already, or something?" Or had the kiss been THAT bad that she'd start in with a girl?