I mostly play Support, so in many games I KS. First of all, the ks are not made in purpose (in most cases), you see a fight and tried to go and help your teammate, so the kill is faster and it doesn't cost too much heal /mana /ultis (and you also avoid the enemy team killing you if they are close, just because you kill him fast enough to escape).
It's pretty annoying constantly recieve bad responses from players when you KS, as a support, i don't kill last kill minions, heal you, buff you, i try to keep you alive, making you to get fed, i'm not saying :
"hey, the enemy champ is almost dead, it's time to kill him, get your gold,screw your score and your perfect game ". It's a team game, so shouldn't matter who gets the kill, it should matter only that the enemy is killed (Of course it's better if the carry gets the kills, but in some fights happens, so don't make a big deal about it).
On several games, some players let me die just because i KS them, so they don't help me in fights, others just spam you, or just blame you on every death the get just because that banana ( yi's jumping against 5 and getting mad at you because no heal or buff, as an example) .
It's just something you have to deal when you play wth suppport chars.
Please don't be childish about ks, everyone can ks, so say ks if you want and keep playing.