Malzahar is a well-known counterpick to Katarina. He outfarms her, so she can't push her lane to go roam for kills. He has longer range on her, so he can harass better. And when she tries to dive him, he just blows his load on her with zero chance to miss his skillshots because she's right on top of him. Building a QSS won't do too much either, so long as Malz notices she's built it immediately (and any good Malz keeps an obsessive eye on enemy builds for QSS anyway). He can just poke/silence/DoT her down until she has to blow it to get rid of the MV, and he can flash in and ult. Malz can only lose if Kat is a much, much better player and he's never played ranked.