Yes, after the SPD unequivocally supported a pointless war that got millions killed, crushed leftist opposition, and teamed up with the conservatives to enact austerity, they did kind of burn their bridges with the KPD. And yes, the bourgeois parties hated the Marxist-Leninists much more than the Nazis who they collaborated with. The results of both of those sets of actions were disastrous.
If only the KPD had been more powerful, not only could WWII have been prevented, but WWI might have been cut short too.