Rex's chronically low opinion of humanity had assumed that there would be something. But Peter seems a pretty good kid, which is very high praise from Rex. He was expecting that maybe he stole something or accidentally injured someone with those powers of his.

He works out that things will be darker than that very quickly and his own smile drops back to his neutral frown automatically. Oh. No no no. This is why he's not allowed at parties, he just drains the life out of everyone and everything and if he was even trying to be fun, to get involved. Looks like he should cut all that out. Look at Peter's face! He rescinds his loom, hands slipping into his pockets. It's an indication of how grim Rex's thoughts are naturally that he already thinks Peter is about to confess to murder.

Which is why he doesn't wince or widen his eyes. He doesn't say a word for a moment, once again showing that tell of his where he touches the beauty mark above his lip. Like it's a button to make thoughts. Eventually, he concludes-

"If you don't want to tell me more I understand," he replies as calm and composed as he usually is, the hot embarrassment of his prior dare cooled by the chilling revelation. He takes care to ensure it's not tentative when he puts one of his gnarly hands on Peter's shoulder. "But based on my knowledge of you as a person, I suspect I don't have all the pertinent information."

Frankly, he doesn't think Peter could actually kill someone. Not as he is. Not intentionally at least and that's hardly killing at all.