Supposedly the reason for his split with the mainline of the KPP is him disagreeing with the social fascism theory and according to Wikipedia (shit source ik) he wasn't even that anti-Stalinist - he still saw him as a socialist, even if one who led the USSR in a wrong direction.
Yeah, he was Jewish (and died over 50 years ago).
Supposedly the reason for his split with the mainline of the KPP is him disagreeing with the social fascism theory and according to Wikipedia (shit source ik) he wasn't even that anti-Stalinist - he still saw him as a socialist, even if one who led the USSR in a wrong direction.
I mean, there were many MLs in the day who misinterpreted Stalin over Social Fascim but I'm not sure about the man in particular.