There are two theories as to why Maleficent turns so horribly evil. In the 2014 Disney adaptation, we find out that when Maleficent was younger, she was romantically involved with the teenage human that will later become King Stefan. In an attempt to win the throne, he cuts off Maleficent's wings, presents them to the king, and when the king dies, accepts his place in the throne. She grows bitter and hateful of King Stefan and the world in general. Due to her hatred of Stefan, she vents her anger on his infant daughter, cursing her to be killed by a spinning wheel on her sixteenth birthday. She also develops a certain amount of sadism telling Stefan that she enjoys watching him beg. However, Maleficent is also very lonely and hurt inside, resulting in her rescuing Diaval, more because she needs a companion than out of pity.
Another theory suggests that as a child, Maleficent didn't have much control over her powers, and was known to accidentally ignite things into flames at social gatherings. (This explains why she wasn't invited to Princess Aurora's baptism.) She becomes so angered at not being invited, as it brings up bad memories from her past, that she curses the child.