The preceding discussions miss the point entirely. The 50 officers killed had already been recaptured, and were in custody. They were undoubtedly murdered as an act of revenge using 'shot while escaping' as an excuse. It was certainly an illegal order and no amount of rationalisation post-event will excuse, or justify, any of the murders done. The word 'Execution' suggests some form of judicial process which did not happen; the recaptured officers were murdered, no ifs, no buts or whatevers; they were murdered as an act of revenge by the German government at the time.
While on the subject of what did not happen, Paul Brickhill's brilliant account of the escape never mentions the character played by Steve McQueen, as it was a compilation character of several British officers' efforts, not American officers, and the motor-bike scene is pure Hollywood fantasy only to sell the film to the US audiences. But unfortunately, it is the first scene that everyone recalls about the film....and he is riding a BSA!