There's something about watching Logan's expression shutter that flares Scott's temper in a way watching the two of them kiss didn't even come close to doing.

Because it makes him scared - for Logan.

He reaches down and gives Jean's hand a reassuring squeeze, but his eyes stay on Logan's face. "I only managed to get as far as me killing Charles. You can take it from there since you lived it, but if you think she hasn't loved you since about fifteen minutes after you met her, you're a bigger idiot than I thought you were."

Uh. Sorry Jean, but he's got a concern. It's not the concern he thought he'd have, but she was happy if confused, and she's not going to be happy watching Logan's expression change. He isn't happy about either one of those. So he's snapping.