On 4 July 1941, Caldwell saw a German pilot shoot and kill a close friend, Pilot Officer Donald Munro, who was descending to the ground in a parachute. This was a controversial practice, but was nevertheless common among German and Allied pilots. One biographer, Kristin Alexander, suggests that it may have caused Caldwell's attitude to harden significantly. Months later, press officers and journalists popularised Caldwell's nickname of "Killer", which he disliked. One reason for the nickname was that he too shot enemy airmen after they parachuted out of aircraft. Caldwell commented many years later: "... there was no blood lust or anything about it like that. It was just a matter of not wanting them back to have another go at us. I never shot any who landed where they could be taken prisoner." (In later life, Caldwell said that his thoughts often turned to one Japanese airman or passenger, who survived Caldwell's last aerial victory but could not be rescued.) A more commonly cited reason for the nickname was his habit of using up ammunition left over at the end of sorties, to shoot up enemy troop convoys and vehicles. During his war service, Caldwell wrote in a notebook: "it's your life or theirs. This is war."