I don't mind if you take one or two kills as the support but...

If you are the support and are not on any kind of carry champ (like Lux or something), it is OK for you to "accidentally" take one or two kills from your ADC, while not great, things happen like where you had to secure it or the would have escaped. But if you keep taking kills from your ADC, and expect to transition well into mid-late game on par with or greater than the opposing ADC, you are mistaken. You might get full build at 30 or so minutes because of the more gold, but your ADC is down 1000+ gold because of it, and the ADC has one of the most expensive standard builds in the game. They need the kill gold because they are balanced (item wise) around that inflated income.

So please, if you have to kill the opponent I get it and it had to be done, but if you are full comboing them when they have >20% health, at least try to hold back a spell or two so you can give the kill to the ADC (or any other role that you are fighting along side). And if you are soloing them, by all means take the kill or try and donate it, this ones up to you.