The obvious kill steal is shooting into burning or wingless planes, unacceptable at any time.
One that occurs when the nme is still fully functional, and this only applies when it's 1v1 and no other nme in area, is when one pilit has worked the nme into a position of indefensibility and you zoom by him and shoot the nme plane up.
I mean, you see your teammate on the guy's 6, he's close in and in need of no help. Then, just because you're in a faster plane or you've got more e, you decide to zoom by and nail the nme plane this other poor schlep worked to get the ad on. I disagree with doing that.
But, in your situation, it sounds like neither fiter has the clear advantage so I would not call that a "kill steal."