The Freikorps consisted mostly of veterans of the Prussian army, and the specific division that executed Luxemburg and Liebknecht was led by Waldemar Pabst a high ranking official in the Prussian army.

Veterans of the Imperial German army who had been ousted from the state military as part of the demilitarization of the German government, yes.

What I am implying is that there was good reason why in the interwar period being anti fascist and anti SPD was not an uncommon opinion.

Except it was very much an uncommon opinion until the Stalinist lurch of the KPD.