AIUI, and I've read a few books but don't claim to be an expert, Wehrmacht SOP for counter-partisan ops in the west was to take hostages and warn the town that if there were any attacks the hostages would be shot. SS SOP was to take hostages, murder half then warn the town that if there were any attacks the other half would be shot.That's why he wasn't prosecuted, not enough proof. Got an excuse for multiple murders in the grounds of Meyer's Divisional HQ?
That would be the US prisoners captured late in the battle, once it was obvious to Peiper that they were losing and he would likely be captured himself? I'd suggest that Malmedy and Wereth were proof of intention.
Isn't it interesting that the German troops were perfectly happy to carry out mass-murder when they were winning, but as soon as they realised that it was time to surrender, it was "no, we never did any of that, a big boy did it and ran away, we only did it because the Soviets were beastly and started first".
Multiple SS units and formations, committed war crimes across Poland in 1939 and France in 1940. Not because they were fragile, but because they thought they could get away with it.