Lend-Lease to the Soviet Union as wartime realpolitik

Why aid the Soviet Union when it had cooperated with Nazi Germany and committed its own atrocities?

Because the Soviet Union was fighting the Allies' principal enemies; supplying it was harsh realpolitik and offered more influence than leaving it unsupported.

Soviet aggression, including its participation in the invasion of Poland, remained part of the moral case against aid. Lend-Lease was a coalition decision made after the Soviet Union became a major combatant against Germany and later Japan. Germany was unlikely to hold a vast eastern conquest permanently while it still fought the Western Allies. Supplying the Soviet war effort was therefore harsh realpolitik that also gave Britain and the United States more influence over an unavoidable co-belligerent.