Of course then Germans, the Prussians and Hessian's especially, had a long history of professional soldiery. Bu Hitlwr’s great move was to make them all swear personsl loyalty to him. And quaint as it may seem a solemn oath by these men was something even higher tan legally binding. Breaking it was not a crime. It was a sin. As paradoxical as that seems with the evil of the war they were conducting, that was the case for many. And once the war starts, you're truly committed.
But they werent conducting an evil war. The things being referred to as evil werent known at the time for their part and the US admitted that we didnt actually know about these things occurring. The US refused to let a boatload dock and sent them away because the US didnt want them either.
None of these things were understood to be real let alone “evil” until the end of the war.