It can get anoying if you let it. Heres my take.
1)On fire does not mean out of combat in all cases. If the enemy is nowhere near anyone else and poses no danger, then killing him is uncalled for, but in a furball, the flamer may have a minute or two in which he is still a threat...so shoot him for gods sakes!
2) Missing part of a wing may not mean dead. If your saddled up on him sure...but Ive landed pleanty of damaged birds.
3) Missing a whole wing, or flopping helplessly in a flat spin is dead.
4) Engine dead, plane dead, inless again, in a big furball and still a threat.
5) Its not stealing when your in combat manuevers with an enemy unless you are directly on their six at 200 yards. I much prefer you help me kill the enemy plane 10 times, than be 'polite' one time when it all looks good from above, but Im running low on E and about to fall out of the manuevers and get killed.
Whenever I do leathal damage to a plane in a furball, I try and call out 'spit dead' or 'La7 going down' This gives the gentle reminder to all would be kill stealers and does something much better, it lets everyone else know they dont have worry about that particular plane. One thing that happens to us all is losing sight of one of 3 enemy planes, and spending valuable seconds making sure he isnt in your blind spot. Those that intensionally steal kills are either not informed or not part of the overal community. The uninformed will catch on pretty quick....the jerk will, whether they know it or not, lose out much more in the long run. There are several people I dont ever help or warn...
viva la KARMA!