When? In 1918 when the KPD split from the SPD? Or in 1920 when the SPD and the Freikorps shot all of their leaders dead? Perhaps when the death of those leaders

When? In 1918 when the KPD split from the SPD? Or in 1920 when the SPD and the Freikorps shot all of their leaders dead? Perhaps when the death of those leaders and the ensuing infighting allowed the Comintern to take influence? No, because Stalin wasn't in power when the SPD and KPD became literally mortal enemies. You don't join forces with people that gave the order to kill your friends. Pretending Stalin was the reason they were alienated is ridiculous - the alienation predated him taking power.