While it doesn't deal with the tensions between the KPD and fascists, I would consider, "They Thought They Were Free" to be mandatory reading on the rise of fascism in Germany.

Even raw footage can be influential depending on how it's edited and selected of course

Calling it a democratic government is quite a short cut. They were in the middle of a revolution, in the process of deciding what the future of Germany would look like, no elections had taken place.

Elections had been declared for the 19th of the same month the uprising happened. Of course, the Spartacists couldn't let that happen. Democracy by the wrong people - the bourgeois elements - is dangerous, after all.

In the population there was huge support for dismantling the Prussian military institutions, but the SPD decided to not only keep the military as is but also to use it against their political opposition.

You're kidding, right?

Much of the action against the Spartacists was done by Freikorps precisely because the traditional Prussian military institutions were no longer available to the government.

I’m not saying the Spartakusbund were the good guys, I’m saying that the excessive violence of the SPD was foreboding of what was to come.

If by that you mean the instability of the government due to pre-existing cultural norms that strengthened far-right elements, political violence, and extralegal action, you are correct.

If by that you mean the SPD were the big mean social fascists crushing socialists movements because they could, as you implied in your original comment, you're dead fucking wrong.