You keep mentioning the Freikorps as though they're some evil force that eroded the Weimar Republic. In reality, virtually no one on any side of the political spectrum had respect for the Republic, but the Freikorps always ended up defending the Republic.

There was a spike in regionalism in some of the German states, particularly Bavaria, who toyed with independence as a means of dissociating themselves from Prussian militarism, war guilt and the reparations that were tied to it. However, far stronger than that were the armed groups of the Left, which sought to replicate the Soviet experiment by seizing power through armed force.

Don't forget that the very same day the Weimar Republic was announced, Karl Liebknecht proclaimed the founding of a Soviet state. This led to the January 1919 uprising, which was only crushed because groups of Freikorps formed to defend the Weimar Republic against the communists. It was only because of them that the Spartacist uprising was crushed. Liebknecht got what he deserved - he initiated the violence and paid for that with his life. Sic semper communistis...

However, in Bavaria the communists seized power and lasted for almost a month of socialist terror before being put down by the Freikorps, who restored the democratically elected forces.

The term is used very loosely, sort of like we would use the term "militia" - it doesn't by necessity mean someone of any particular political persuasion. Indeed, the record of the various Freikorps units skews wildly all over the spectrum, with some groups scrupulously defending order and others pushing extreme agendas. Some were anti-Semitic while others welcomed Jews. Historian Thomas Weber did a good analysis of just what happened in Munich during the revolution, who supported whom, how Hitler tried to coordinate with the Kapp Putsch only to realize it had ended by the time he got to the area around Berlin and what the general political currents were in his book Becoming Hitler (note: I read this book in German because I found it on amazon.de and wasn't aware it had been translated until I got the book and saw the translator's name, though I'm glad I read it in German because there are copious citations from letters that were restored to their original German).

If I had to point to the downfall of Weimar, it was that no one really liked it or wanted to save it. Everyone was seeking to resolve disputes in radical, violent ways and everyone was looking for a strong leader to push whatever agenda they had. The economic hardships, combined with the polarization of political views that inevitably resulted due to the internal pressures on society from those economic hardships, made half-measures seem weak rather that conciliatory. For the US to get to that point we would have to have another major economic meltdown. Also, note that it was the Left that first resorted to violence all across Germany. The right began to fight as a self-defense strategy.