Rengar thrives on forcing a player to go massivly negative when he is doing well

How is this allowed? I mean it's all good and all that an assassin with their own ability to think "okay, now is a window of opportunity to go in and smack that ADC up and kill him" but holy shit there is no point where Rengar does this after he has 2 items built, it's "Oh my ult is off cooldown, where is their ADC/Immobile mid so I can give them a second shithole"

/counterplaypls