There's also the worry that US involvement kept the Soviets in power. Richard Maybury suggested in his book (and I doubt he's alone on this) that both the Soviets and the Nazis were about to collapse in late 1941, and American involvement helped keep the Soviets in power.
[Besides, if one were pro-war and disliked both sides, wouldn't it have made more sense to pounce on whichever survived that war in their weakened state?]