You can not award the kill at the time of damage. The outcome of the flight has still not been determined. The odds are he will crash and die,but that is not always the case.
But I have lost wings at times near ground and ended up with a ditch by luck.
What would be possible is that no more lethality points are tracked once certian componets fail. The plane could still be shot and more damage done, but no kill award tracking would be done, you still must stay living until the hit plane exits flight.
Been thinking about implementing this for a while, it just hasn't moved to the top of the list yet.
HiTech
That would fix the kill-stealing. But there's still the problem of times where you inflict the fatal damage, but still don't get the kill because someone else scattered more non-damaging hits on the target. Happens to me all the time against Spitfires. Short burst rips off his wing (or if I'm close enough, BOTH) or I put a single .50cal round through his cockpit when he has no other damage (no PW, no fuel leak, no smoking engine, no missing control surfaces, gear, radiator or any other component) and I still only get an assist.

IMO, the "kill points" should only be applied when component failure/damage occurs, and different components should be weighted differently. Say, any sort of clearly fatal damage (PK, horizontal or vertical stabs, whole wings, rear fuselage, etc) is worth 30 "points," a PW worth 20, half a wing is 15, with the other components going down from there depending on how significantly it affects flight. And damage points will be accumulative. Blow off his whole wing, and if the wing was intact you also get points for all the components attached (guns, ailerons, flaps, gear, etc) as well. However, if that plane is already damaged by someone else when you do (say, gear and guns shot out on that side) you only get the points for the wing and whatever component is left. That way just HITTING the target doesn't count. You have to actually knock something off of him.
If the damage model ever gets more detailed than the "All or Nothing" system we have (so, put a bunch of holes in a guy's wings and he may just not be able to generate enough lift to stay in the air even though the main spar holds them on the plane) points could be used for hitting the target while still weighting component failure. But as it stands, it's frustrating to cause fatal damage with a lighter weapon, only to lose the kill because someone else landed a 30mm round even though it caused no damage whatsoever a half hour earlier and the shooter isn't even in film viewer tracking range (I've had that happen, too. Guy's about to land a sector away and gets the kill on someone he hit but failed to damage when I blow his wing off).