10k Damage Rengar Right Click?

So, I get that URF is about imbalance, and that's fine, but when a champion does something with only 1 use of something, it's not URF making them OP, it's just them being OP.

I played Nasus, built AP Tanky. I had like 120 armor and 4k health, so not bad. Rengar at the end of the game with Duskblade was able to deal 6k damage instantaneously and one hit me with a single auto attack. Through 120 armor. He had some lethality, which is okay, but considering that with just a Youmuu's and Duskblade the most he could get it down to was 84 with 36 lethality, that means he was dealing a total amount of 10k damage, if he were dealing true damage.

That's literally enough to one hit a stacked Cho'Gath in a single attack. I just don't get why some champions are allowed to be "lul, I got 2 kills early, gg" champions. You feed first blood to Rengar and you and you entire team may as well just Alt-F4 because your chances of winning the game just dropped to like 4% at best.