Although this subject has been beaten to death for the next 200 years I gonna add my pudding to it!
I cannot find anything realistic in a system that promotes cutting in ones way to steal a kill (plus killing the pursuer while taking a shower in his stream of well aimed bullets).
Realistic solution: Bullet/shell does equal damage enemy and friendly
- Pro: as IRL
- Con: possible abuse by some freaken kiddos; punishment system (e.g. PNG) hurts if friendly fire by accident (or by stupid killstealer)
Toned down realism: Dmg to friendly at a lower scale (maybe 1:5 or so) plus introducing punishment system a la PNG*.
- Pro: almost as IRL; friendly fire by accident less probable - friendly fire on purpose will be punished hard
- Con: still possible abuse
*Persona Non Grata: one you wouldnt trust --> e.g. no live ammunition

Alternative punishment: harsh perk point reduction/ plane restrictions/ "dummy"-icon (

)/ <-- maybe combinable with "free bounty" e.g. NO punishment if you shoot the baddy down
No friendly fire
- Pro: no stupid deaths by killshooter anymore;
- Con: killstealing could be a problem (but at least the pursuer in the first place isn't punished for it
Besides "selfkill" for buffs would enchance realism as well. Shooting through your own fuselage/wing is really kinda silly (well 1.10 promised "cold-trigger" zones)