Someone who's ill-natured is cranky and disagreeable. Most fairy tales have at least one ill-natured antagonist, an evil stepmother or a ferocious ogre.
Mean and nasty people are sometimes just plain ill-natured — it's not in their nature to be generous and cheerful. Your ill-natured cat probably scratches anyone who tries to pet her. Bad kitty. This unfriendly adjective combines ill-, "badly" or "not well," with natured, from nature and its "innate disposition" meaning. The Latin root is natura, "natural character or quality."