It's not so much about after he got in power, but how his personal history drove him paranoiac and sociopathic, wanting to hide from the previous government, with the revolutionary mindset and culture of underground and secrecy, with collusion with crime.
But all that is good. What is not good, is that he got into power. Stalin was right as a citizen. But the problem was that there was power for him to take, and as a citizen who understood government, he used its power to the full extend. What was bad was not Stalin, but the fact that there was power.