Any horrible person who came to America and looked at both parties would probably find a ton more to identify with in the GOP.
One of the things that is consistent in all assessments of Stalin from his youth until he had secured power is his lack of respect for his superiors, contempt for intellectuals and hatred of people with more education and/or intellect than himself, and being a bully (he was involved in lots of street fights and a gang member in his youth).
I think being anti-intellectual and a bully makes him more likely to be Republican, the anti-authoritarian streak could be Democrat, but given that Stalin was more respectless and rude, than genuinely opposed to authority this is a bit dubious.
He despised religion and ignored his family after leaving home and was cruel towards his own family when he established one - so no family values, which may signal Democrat, but also very career minded and egotistical which could be a conservative trait (all though plenty of liberals share those traits),
On balance I think Stalin would be a Republican, but one who despised the religious types and genuine SoCons in the party.
So basically the kind of Republican who's a social misanthrope who listens to Alex Jones and probably reads Ayn Rand and Men's Rights forums.