Bolsheviks.
The last Tsar was a scum bag. He was the richest man in the world at the time of WW1, yet vast majority of Russian people were among the poorest in the world at the time, starving and living in hardships for generations. He ruled with an iron fist and clamped down on the lower classes when they asked for reform in policies to better their lives. He got these poor and starving peasants to fight disastrous wars for him, which they ultimately lost. Wars where they had little to gain for the people and where soldiers were under equipped, poorly planned and suffered high casualties. This put the country further in debt and the burden picked up by the lower classes, after losing their sons in the wars. The revolution was an accumulation to a breaking point.
https://en.wikipedia.org/wiki/Bloody_Sunday_(1905)
If I was transported to 1917 Russia then I’m picking up a gun against the Tsar on side of the Bolsheviks.