When you're maleficent, you're evil or want to do evil things. So it's safe to say that Satan is one maleficent guy.
This word is a bit old-fashioned and you don't hear it often. But there are still plenty of maleficent people in the world — they're the bad guys, the evil-doers, the folks who wish harm on others. Maleficent means something very close to malevolent or diabolical and is a lot stronger than naughty or mischievous.