Killer Cat is an enemy appearing in Day of the Cats Event. Completing this event will unlock its Cat's counterpart, Killer Cat (Rare Cat).
This is basically a watered-down Crazed Cat with a Black attribute, but it gains a strength magnification as the player progress through the Event. Luckily, it only attacks one Cat at a time(single target), but spawns in huge groups in the last stage. To beat them, you can outlast them as their spawn rates eventually decrease to a mangeable level.