Nuking the Kremlin in 1945 would have saved millions of lives in the second half of the 20th century. That would have ended the murderous gulags and Soviet genocides, no communist China supported by USSR, Korean War, etc.
Problem is, it would have set a poor precedent, that eventually would have destroyed us down the road. Maybe not right away, but eventually.
If history has taught us anything, is that you get rid of one baddie, only to see another one rise up.