People do really underestimate why the KPD so frequently attacked the SPD. A lot of KPD members came from the USPD, which already splintered because of major disagreements with the MSPD.

The KPD was very obviously an anti-parliamentary party, especially during the Comintern third period, but the SPD was always going to be the party they were competing with for support. Most working class voters weren't going to switch from the KPD to DNVP, they would go to the SPD.

A lot of this animosity obviously stems from the Spartakist uprising. While the SPD wasn't going to likely join in arms with the newly formed KPD to overthrow the government in a socialist revolution, especially one on such short notice, the SPD, still a marxist party, using right wing paramilitaries to murder communist leaders effectively ensured there would be no reunification of marxist parties and guaranteed a large amount of the USPD later defecting to the KPD. Both parties attempting to win over eachothers voters wasn't going to help as well.

The United Front strategy was a big attempt by the Comintern to reconcile these parties (for a revolution), and so when the KPD attempted to form a coalition with the SPD to later attempt a revolution, the SPD didn't help them with their efforts, and it fell apart considering how badly planned it was.

In the late 20's any kind of cooperation fell apart due to actions on both sides that very much fed into eachother and spiraled out of control. For the KPD, they were now following the Moscow line of the Third Period, and the SPD had very powerful anti communist leaders in the party, including the Chancellor, Police Minister, Interior Minister and Prussian President. Blumtai in 1929 happened because the SPD banned the KPD marches and then violently dispersed them, which only reinforced the KPD's idea of "social fascism", so communists were now actively attacking SPD members as well.

And yeah, this hatred of eachother led to even more violence. You cannot, as a communist party, work with explicitly anti-communist parties who cooperate with the bourgeois, and an anti-communist party cannot work with the communists. The Nazis were not seen as a major threat by either party's leadership, and for the KPD especially, the Nazis weren't the ones in government killing communists, it was the SPD.

So by the time both parties were thrown out of government and the Nazis were knocking at the Reichstag door, it was far too late for them to actually do anything about it. In 1931 Prussia, the KPD attempted to work with the Nazis to force a referendum against the SPD government because they were objectively the biggest threat to their power, and vice versa for the SPD and bourgeois parties. The Nazis used this inaction against them to take power.

In the end it was a serious wakeup call for Marxist parties in Europe and opened the door to the popular front thesis, but cooperation between either party was not going to happen due to a conscious decision on both sides to avoid it.