Of Commonwealth and Desert Air Force pilots Clive Caldwell is known to have shot at least one German pilot on parachute. When asked later why he did that he said that he saw not any reason to let enemy to live. The usual justification of killing parachuting enemy airmen has been that they will be eliminated to take a new plane and going to fight again and possibly kill the ones of your side. This applies specially to cases when the enemy is parachuting to safety over his own territory.
However in some cases parachuting pilots have been shot in their parachute even when they were parachuting over hostile territory and thus likely to be captured. At least one such case happened during the Soviet-Finnish Winter War in 1939 when Finnish pilot was killed by Soviet fighter when he was parachuting over Soviet territory - the case has been recorded by Soviets. Totally unnecessary kill as the Finnish pilot would have been captured anyway.
The most systematic killers of parachuting enemy airmen were Japanese fighter pilots - of this more on:
http://forum.12oclockhigh.net/showthread.php?t=38310
When it comes to Germans, there have been claims that Legion Condor pilots did strafe the parachuting enemy airmen during Spanish Civil War. However I do not know details and if it was systematic.
The Italians and Spaniards of both sides of Spanish Civil War were quite chivalrous - they usually left parachuting airmen in peace as far as I know. In one famous case one Spanish pilot even escorted the damaged enemy plane so long that it could make a safe forcelanding over its own territory! Sometimes they also escorted the parachuting enemy pilot circling around him until he reached the ground, greeted him and then flew away.