15... 30... 50 kills, all the same really. Past like 10 they don't really matter, they are not points and this game doesn't use points
Secondly. Most any champion seems pretty OP once they get like 8 kills, and ahead of other players. I have gotten 5/1 as Caitlyn and then 3v2ed with the top laner who was the furthest behind, and won without taking any casualties. It's why I wrote the first point, League is pretty snowbally so the kill number stops mattering pretty quickly.
As for Rengar, he can be strong. He can even get to a point where his abilities can be "unfair", jumping on an slaying a champion before they react like you saw in your match; it's why he has a reputation of being popular with douchebags.
But Rengar, as a champion, does pay for that by being a champion that if you are not pulling yourself far ahead of your enemies and are slaying them in one shot, you are falling behind, becoming deadweight and having to play as the most worthless tank in the game.
That is, if Rengar is not "leaping into and instantly slaying his enemies", he is "leaping into and instantly feeding his enemies".