Looking in wrong direction? I haven't read the complete thread so someone may have said this already for "top American Killer".
In ETO it was probably an unnamed U Boat commander and in the pacific the Jap that sank the Indianapolis is probably up there and the commandant POW camp in Phillipines or Tokyo ranks up there as well.
The fighter pilot is way down the list I would suspect
The top B-17/B-24 shooters are probably the best ones to look at for LW, depending on matching the KIA to the pilot. Egon Mayer for example
Actually no, and at the end of the day my example was one man with a team behind him - as a U-Boat Commander.. whereas an Egon Mayer was in a single engine airplane (or Eder, etc) and flying all by his lonesome...