Flight Instructor reported observing an aircraft taxiing onto the runway with another aircraft on short final requiring evasive action. Pilot also reported the taxiing aircraft failed to monitor on CTAF.

Narrative

We [Aircraft X] were in downwind and observing the event that occurred at RAC this morning. We were following Aircraft Y (who had to ultimately make evasive action). Aircraft Y was on a very short final (100 feet AGL according to our ADSB report) when Aircraft Z announced on CTAF that they were taking the runway for departure right in front of the landing Aircraft. The Aircraft Y immediately went around; making an immediate climbing turn away from the runway environment. The Aircraft Z continued down the runway before stopping. At this point we were on our base to final turn. When our wing was out of our visual of the runway we saw the Aircraft Z now back taxiing down the runway. We were required to reduce our speed to our slowest possible speed to make room for this Aircraft to now vacate unannounced. It was clear the pilot was not paying attention to the CTAF frequency when he took the runway and then did not make appropriate calls when aborting his takeoff. This forced both Aircraft in the pattern to either make a low; slow and immediate go around or reduce speed significantly to accommodate their actions. I wish I had an answer for how to improve non-towered airport operations; but this was such an egregious error on behalf of this pilot I felt it necessary to at least report.