The King in Maleficent is King Stefan, and is played by actor Sharlto Copley.
The character of King Stefan first appeared in the 1959 animated version of Sleeping Beauty. He is the father of Princess Aurora - Sleeping Beauty herself. After the evil fairy Maleficent curses baby Aurora by saying she will die on her 16th birthday by pricking her finger on the spindle of a spinning wheel; King Stefan and his wife, Queen Leah, must make the difficult decision to have Aurora raised in the forest by the Good Fairies to keep her safe.
The version of King Stefan in Maleficent is quite different. He is a ruthless, power hungry king willing to do absolutely anything to remain on the throne, even if that means killing Maleficent.
It is suggested throughout the movie that his cruel actions may be what turned Maleficent evil in the first place.