The main thing that Stalin did, that I find very repulsive, is allowing regular people to be labelled "enemies of the state" and similar based on hearsay, gossip among neighbours and reading private correspondence, then sending them to labour camps that were so harsh that many died.
I do not know your skills in Russian but I suggest you to read this document signed by Stalin and Molotov:
Молотов В.М., Сталин И.В. Об арестах, прокурорском надзоре и ведении следствия: Постановление Совета Народных Комиссаров ССС