Not all Communists are Marxists, but all Nazis and Fascists are Marxists - even if they are in denial.
All of them promote evil, destructive ideologies rooted in the same fundamental errors.
All socialist and communist factions were and are permanently at each other's throats. In the USSR, it started with Mensheviks vs Bolsheviks, then Stalin vs. Trotsky, Fascists vs. Italian Socialists (Mussolini had been a life-long member of the Socialist party before creating the Fascist party), the National German Socialists Workers Party vs. the Communist Party of Germany (Hitler had been a member of the Communist party before co-founding the NSDAP), Mao's China vs. the Vietcong, the Vietcong vs. the Khmer Rouges etc.
It's a bit like the different factions of Islam which have been fighting each other since Mohammed's death.
Totalitarian ideologies will always attract power-hungry psychopaths who will fight each other for control.