Somehow, through possible butterflies, make it where Stalin and Hitler DON'T become the mass killers they were in OTL. Whether they still enter politics or not is your choice, but here are the limits for either.
Hitler: Can't be part of a far right party. Its okay if he's a conservative, but no radical right party. Radical left can be okay, but again, no massacres, purges, or that sort of thing.
Stalin: Can still be a Communist, but not a murderous one. Unlike Fascism or Nazism, killing isn't inherent to Communism, hence can pass the test. However, you probably want him to avoid radical politics, unless through that you can make him not so murderous.
Now, these people can still kill, but it can't be anywhere NEAR the level they did in OTL.