In the context of the film the Basterds are simply giving them their just deserts.
Plus, it must be remembered that this film happens in an alternate universe. It doesn't have any bearing on our history.
It was not possible to commit war crimes on Nazis.
Alas, it does. That is why the traitors in Hollywood make movies that celebrate traitors.
Check “Bridge over the River Kwai” where the honorable British commander, who’s men were being murdered by the Japanese in a disgraceful fashion, yet still resisted, was presented as a collaborator.