Nazi collaborator
If he fought in the SS, that's not a Nazi collaborator, that's a Nazi. There's no semantics game to be played, that's outright a Nazi.
here's an excerpt from a speech that Heinrich Himmler gave to the division this man served in, i.e. he was most likely in the crowd. CW: antisemitism, anti-Polish racism, genocide
Your homeland has become so much more beautiful since you have lost – on our initiative, I must say – those residents who were so often a dirty blemish on Galicia's good name, namely the Jews ... I know that if I ordered you to liquidate the Poles ... I would be giving you permission to do what you are eager to do anyway.