What is reported to have taken place on board the aircraft is truly heroic. In their desperate, no-escape circumstance it was only sensible to 'have a go'! I too wish the passengers and cabin crew had succeeded.
May God rest their souls in heaven and grant peace to their grieving families and loved ones.
But is it not probable that coincidently the aircraft was shot down to ensure it would not be crashed into another significant national target building with further huge loss of life?
The reported distance between the tail/empennage, and the main aircraft fuselage/wings impact locations points to a mid-air breakup. The expanding shaped charge shrapnel pattern from an exploding air to air missile(s) would be capable of cutting the aircraft about that point, resulting in the mid-air separation of the fuselage, hence the impact separation distance.
Knowing what had transpired in New York and Washington, plus knowing the on-board status and intentions of the hijackers, had I been the fighter pilot I would have also obeyed the terrible but necessary order.
Understandably the President has placed a news-ban on that event.
So, if my notion is correct I wish the fighter pilot in question Godspeed in accepting that he HAD to shoot the aircraft down, to minimize horrendous collateral damage; and may he be at peace within his conscience - having done his duty.