One should watch "The Act of Killing", which goes into the 1965-66 purge of suspected "communists", resulting in a million deaths. Even though Suharto is gone and Indonesia is now a "democracy", the legacy of the killings is still strong, the perpetrators are celebrated as heroes and the country is pretty right on the Overton window. It was sobering and definitely made me reexamine my own thoughts on "Authoritarianism". The enemy won't give a damn if you play by their rules.

On a side note, look up Iran's Tudeh, which has a history of casting their lot through electoralism/aligning with reactionaries and getting screwed over twice respectively (in 1953 and throughout the 80s). China is one of the rare exceptions where the Communist Party was able to bounce back after getting stabbed in the back by liberals and reactionaries.

This isn't to say Leftists shouldn't form united fronts with non-Leftists when its practical to do so, mind you.