Bader wasn't fooling around at the time he lost his legs- he was displaying another human trait- pique! He was pissed off at being goaded into giving a display of aerobatics, and let his temper overrule his better judgement. One major rule in flying is not to do that. Had he just been "fooling around", he would have been more relaxed and at one with his machine, and most likely have waited for more airspeed and altitude to show off... And had he still had the previous aircraft type he had flown, he more likely would have not had the accident- they were much more forgiving of such maneuvers... But then again, had he not lost his legs and been discharged, he would have probably been a Group Captain by the start of the war, and really would not hve shown as he did, being higher up the food chain!