Riot, Katarina was balanced before.
Remember when Talon, LB, Kassadin had silences? Remember when poke champions were strong because athene's was actually a good buy? Remember when diana was strong? Or Kayle?
You nerfed SO many champs and items in this game, that katarina finally became viable. Before that , she was actually pretty weak, and even needed buffs ( since you buffed her ultimate ). If you keep nerfing her, soon, champs will do 0 damage to each other, because a new viable mid laner will appear, and get nerfed. Stop with this idea of only nerfing, and instead give small buffs to older and weak champs