Michael, I know that you meant “Eichmann” in the generic sense, but it’s worth pointing out that when that monster was captured by the US Army he was operating under a pseudonymn. The Americans had no idea who it was they had really caught. Subsequently, he escaped. If his identity had been revealed back then in 1945, I have little doubt that he would, deservedly, have gone to the hangman then and there.
On the wider point as to whether it was morally acceptable for the West to enlist ex-Nazi military, scientific and intelligence personnel in the struggle against Soviet totalitarianism, the answer must depend on who was recruited and for what. In a desperate struggle, the good guys may occasionally need to recruit some very ugly allies. Churchill had plenty to say on this topic after Hitler invaded the USSR in 1941. Besides, viewing all former Nazi party members as identically evil makes not much more sense, morally, politically, militarily, or historically, than the decision, after the fall of Saddam, to treat all Baathist officials as equally reprehensible and thus equally unemployable. The western allies understood these issues in 1945, but those lessons appear to have been forgotten by the US part of the coalition in 2003. That was a mistake that appears to have made a significant contribution to the current problems in Iraq. Incidentally, while I certainly wouldn’t use Stalin’s Kremlin as a moral exemplar for, well, anything, it’s also perhaps worth noting that the Soviets do not seem to have had qualms about recruiting their own Nazis in their post-war assault on the West.
In weighing these matters, it’s also important not to view hard, hideous choices made in 1945 through the lens of 2006. Failure to lock what became West Germany into the democratic world would have been a strategic disaster, a disaster of an extent quite sufficient to hand the feeble and broken Western Europe of the immediate postwar years over to Stalin (remember that the US had demobilized very rapidly). Contrary to what is now being argued, back then it was by no means clear that the future West Germany was going to evolve in a decent direction. To take one example, surveys taken by the Allied forces in Germany in 1946-49 revealed the horrific fact that anti-semitism persisted on a “massive” scale, reason enough then to be worried about what the future might hold.