I wouldn't consider this whinning because I think its common frustration in the game.
I admit, I have ''stolen'' some kills in the past but never really intentional. For example if I see one of my guys fighting in 1vs1, I'll stay out but will ask if he needs help. Usually because busy, I don't always get response right away. If that happens, i'll wait (if no other baddies around) and if he can't shake the bad guy off his six, I come in and make attempt to clear his tail.
Another example is when I see several guys chasing bad guy. I'll dive in to help turn the baddie around, although sometimes I end up shooting him down and get the kill (this goes under stealing unintentionally).
There as been times when I will help one of my guys shoot down someone if they are having hard time; I'll score some pings and then disengage, hoping that I did enough damage so my guy can get kill but doesn't always happen that way (this is pretty similiar to the example above)
This is more of an example of gangbanging but I got PM from one of my guys claiming I stole the kill. I won't say his name or the name of the bish, but I dove down to clear off YAK on this guy's tail. I was coming in pretty fast but got good hits near engine so made it leaking oil which in turn made him turn away from my guy. Few more knights came in and end up being 4 vs 1. I didn't want to particupate (please excuse my bad spelling) so I disengaged and went off to find my own fight. Few minutes later, the ''you shot down so-so" came up. That was when the PM came.
I simply replied back and told him that he was in fact trouble and all I did was dive down, few hits then dove up and away from the battle so he could get away, or get on yak's six