We have to understand the knowledge vacuum many Russians were operating on during the war. Neither the Soviet nor Nazis were open or honest about information sharing, and what is more important to recall, many people had already realized this even in the early war, leading many Russians to distrust reports of fascist atrocities. Many Russian soldiers felt betrayed by the Soviet government, either from the peacetime brutality of the collectivization and purges, or from the horrific mismanagement of the Red Army during 1941-43.
As one great and emblematic example, Vlasov had led the 1st Shock Army during the winter counteroffensive of 1941, and he was a victim of the aggressive but foolish overextension forced on the Red Army by Stalin's obsession with the offensive. Moreover, Vlasov was furious at the lack of logistical support given to his army once the winter counteroffensive turned sour, and the inflexibility of the Stavka which refused to allow tactical retreats (a situation the Germans would themselves experience in the later war). Add to the unrest amongst the officer cadres the mass illiteracy and political apathy of many of the rank and file. Anecdotes report occasions where Russian POWs simply asked about comparable ration, vodka, and tobacco allotments in the fascist military. It must also be allowed that many in the ranks had hardly been committed communists, and many were survivors of political intrigues or Holodomor.
But as important as any ideological or unpatriotic motivations amongst the soldiers is the fact that the decision to join the fascist military was hardly a fair contractual transaction, but more honestly a reflection of undue influence or duress. Russian prisoners were starving in the Wehrmacht's prison camps by the million, and conditions in the requisitioned countryside were hardly better. Many Hiwis doubtless joined out of desperation, victims of a brutal war that did not allow neutrality.
Because germans mistreated soviet prisoners of war, according to german sources, from 50 to 60 % of all soviet war prisoners died or were executed during the war, with a total number of prisoners from 3,5 to 5,25 million people.
So my guess that for a most of them it was pretty simple motivation: to live as a traitor (with a chance of redemption, because after war only few of collaborators were severly punished) instead of die in a concentration camp. Sure there were people, who hate USSR, lots of nationalist scum etc., but nevertheless.