Hello Bryan
If I recall correctly W/Cdr Clive Caldwell gained his nickname "Killer" for the regular practice of shooting at Axis airmen who had baled out of their aircraft. I have read that he did this in revenge after witnessing a German pilot do this to his friend P/O Donald Munro on 4 July 1941. Other accounts have him being bestowed this sobriquet for his ground strafing activities. Perhaps others can add additional information confirming or not any of this. An interesting article on him can be read at:
http://goliath.ecnext.com/coms2/gi_0...me-Killer.html.
Horrido!
Leo