You can describe someone as devilish if they're nasty and cruel, although this adjective is also used for rascally or naughty people, like the devilish kids you babysit.
A devilish punishment is cruel, but a devilish preschooler simply misbehaves in a playful way. Since about 1600, the word devil has been used to mean not just an "evil, diabolical spirit," but also a "clever rogue." The adjective has followed suit, so you might describe a terrifying movie monster as a devilish creature, while also calling your favorite rascal a devilish creature.