Get over yourself. It's a problem that is as old as multiplayer shooters. Are you really wasting time reporting people for something that has probably occurred by accident in 99% of matches since the games launch?
Is it too much to give them the benefit of the doubt? What if they just don't like leaving anything up to chance? They might have assumed that you wanted the win just as much as they did and nothing more.
I'm not going to hold off on a safe shot when I see it just so that I can say I was courteous. That does nothing for the present situation. If it might stop my teammate from taking one more hit it's worth trying. If I kill them in one shot, oops. Chances are I didn't know their exact health anyway.
With experience you will see that kill steals are something that no shooter has managed to properly circumvent. It's just a reality of playing the genre. You can't enjoy it if you can't get past it. It all comes full circle when you're the one finishing the job, which can be a lot if you make sure you're helping in every battle.
