Ref to Guilbert de Corbeil:

"Stalin was an ally of the US..."

From about 1941 - 45 that's true.

From 1945 until his death Stalin was not an ally of the US.

The clearest examples of this is Stalin giving the green light to Kim IL sung to invade SK in 1950 as well as the blockade of Berlin which accelerated the Cold War.

Who killed more people, Stalin or Hitler?

Hitler killed about 7 million Jews, gays gypsies, and political prisoners in his concentration camps, about 5 million Allied soldiers, about 1 million Allied civilians and at least 21 million Soviet civilians (probably more).

Stalin killed about 5 million fascist soldiers, about 500,000 German civilians, about 10,000 Polish officers, and about 10 million Soviets he deemed as undesirables.

Both were totalitarian monsters.

But Hitler certainly killed millions more people than Stalin did.