you are quite right, sir:
The partisan attack was led by the GAP Gruppi di Azione Partigiana and resulted in the deaths of 33 German soldiers.
In retaliation, the largest cohesive group among those executed were the members of Bandiera Rossa (Red Flag), a non-mainstream communist (Trotskyist) military resistance group
In addition, Priebke was ordered to round up the partisans by his superiors after the 33 German soldiers were killed.
It is not as if Priebke went “rogue” and did this all on his own accord. Priebke was not a major criminal and the death penalty would not have been appropriate in this case.