Traditionally the reason for not allowing enemies to escape is that they will regroup and fight you again (accepting surrender, or allowing parachutists to land & become POWs, is a different matter). In this case, the escaped crew was able to return to bomb German civilians.
How unfashionable by today's standards of shooting a guy in rags from IR vision enabled, high calibre helicopter gun because he has old rifle and some ammo so might pose a problem to somebody at some point in the future.
Good is good. Evil is evil. The only way for there to be more good is to do more good. If you do good and others do evil, you have still done good. If you do not do good and others do evil, there will be more evil than good. If you do good and others do not do evil, there will be only good done.
As a German in WWII, Stigler was a modern Samaritan- spit on for their acts, Nazis acts were and are despicable- just like the Samaritans were to the Jews 2000 years ago.
Is there any reason for that?