For how much Greg was just thinking it's good to talk through your thought process and figure things out, he's too tangled up in his confusion to do the same. This is one heck of a thing to learn about someone you thought you knew, with them standing right here, waiting for your response.

Who was this kid? Why would Sans hurt them? When did this happen? There has to be reasons, this can't possibly exist in a vacuum, maybe Sans is a bit of an asshole but he can't just murder someone for no good reason, that's not something people do.

It's pretty twisted, trying to justify why someone would kill a kid. That's got to be why Sans is such a wreck, too. Or, part of it.

He's never seen Sans like this, he realizes. The gibbering, nervous wreck, obviously, but he hadn't thought much of it a minute ago. They'd all seen some serious terror tonight, something that none of them were likely to forget anytime soon. Being erratic and dazed and frightened made sense. No, Greg realizes, that's not the thing that seems so completely new about Sans.

"Wow. Sans, do... do you know what this means?" His laugh is a tiny, sad little noise. "You're finally... really being honest with me."