You have to realize that every part of russian society was divided during the civil war. There was cossack, peasants, criminals, urban worker and foreign soldiers on both sides. Only the Nobility would be mostly fighing for the whites for obvious reasons.

A good part of the common foot soldier, if not politicaly motivated, was mostly coerced to fight for one side or the other. Some villages or regiment (this could often be the same thing, as recruitement was sometimes closer to raiding than anthing else) fought for both sides during the war. Recruitement by the white army was done almost like medieval levy, forcing recruitement of the local population of controlled territory.

The communist mostly controlled urban center at the beggining of the conflict, so a lot of their initial manpower was urban workers and desertors from the now dead russian army. They too relied soon enough to forced recruitment (often playing the food hoarding card) to boost their manpower.

The third forgotten participant, are the people, taking arms, who did not want to join any side or see some opportunity from the mayhem . They were loosely referred as the Green army, and it was mostly a patchwork of civilian militia trying to stay (or become) independant, anarchist army, and raiding bandit. They would also change side often or try to stay independant, adapting fluidly to the situation.

This entities changed size and composition as their goal changed. For exemple: a small militia could be created to protect a town by its inhabitants, then be included in the whites army when a strong loyalist forces arrives in town claiming ownership. After a loosely organized skirmish far from suplly line, the milita turned regiment could escape from control and go free, raiding the countryside for survival, adding desertors or desperate peasants to their rank. After some times, the survivors of the initial militia could end up fighting with the Red Army, because they were captured along the way or had some political epiphany...

The green "peasant" army were also the last forces opposing the red army at the end of the civil war, months after the white has been defeated and evacuated.

And just to get back to the beginning of your question, regarding "the reds being criminal". Most communists were criminal from the tzar and nobility point of view only, because of their ideal. They were not a chaingang of convicted murderer and rapist...

TLDR: People from all kind of background joined or were recruited on every sides of the russian civil war. Even people who did not want to get involved had to choose a side at the end. War is horrible.

Source: "The russian civil war 1917-1922" Jean-Jacques Marie.