AAQ: “I would be careful claiming that Rebbe killed Stalin. Why didn’t here do it earlier before millions of people were killed?”
The story with Stalin unfolded at a farbrengen , it’s no secret, you can look it up easily I’m sure
The Rebbe is fighting Hashem’s battles and cannot do anything before the time Hashem wills.
A deeper reason could be that The Rebbe wasn’t appointed as moshiach yet before then.