Kwaad could be both a noun and an adjective! However, it has a different meaning. Kwaad as a noun means 'evil', while 'kwaad' (or 'kwade') as an adjective means 'angry' (when you are 'kwaad', you are angrier than 'boos', but not 'furious' yet).
Oh... What is the adjective for evil, though? I checked Google translate and it gives me boos/booze. I guess that works for German if you spell it correctly (böse), but does boos/booze not mean evil in Dutch?
Evil, like a villain (kind of), right? I would say: 'kwaadaardig(e)' or 'boosaardig(e)'.