Joachim Peiper - he didn't execute Americans. Some of the troops in his kampfgruppe may have.
It was French Communists that killed him. You must remember that French Communists also eliminated people in the French Resistance who weren't Communists whenever they got the chance - in order to clear the way after the Amis threw off the Nazi yoke.
There was a history written in Germany a decade or so ago and it was pretty clear that he might not have been present, nor given the direct orders fro the slaughter at the cross roads, but he did inculcate a certain brutal attitude which lead to such acts. There was at least one of his soldiers who testified he gave a direct order to kill a prisoner at another time.
He was released from prison in 1956/57 not because he was innocent, but because the court case had been screwed up when torture was used by American MPs to extract confessions from the enlisted soldiers at the slaughter, included crushing genitals. As such the evidence against him and the other leaders was flawed, and it was felt that late, 12 years (1956) later it would be impossible to get a fair trial, so they were let go, Pieper last of all.
While he might have possibly been innocent of the exact charges, he was not a very savory character. In particaur he was Himmler’s aide in May of 1940 and had to know about the einsatz commandos, the 3 or 4 groups that followed the German Army into Russia and killed civilians.
There is more but I think the above is sufficient to make the point. The US attempted justice, even when dealing with NAZIs.