Due to hundreds of years worth of evil deeds, Mr. Gold/Rumpelstiltskin (Once Upon a Time) suffers from a heart clouded by darkness, his heart being consumed by it until there was nothing left but the darkness.
King Sombra (My Little Pony: Friendship Is Magic) was the tyrannical ruler of the Crystal Empire "whose heart was black as night."
Kid Buu (Dragon Ball Z) is a being with a pure heart, a pure evil one, as evidenced when he repelled the Spirit Bomb, which only those pure of heart can do.
Cell (Dragon Ball Z) is a being with a heart of pure evil, as evidenced by his usage of the Spirit Bomb.
Mira (Dragon Ball) is a being with a heart of pure evil, as evidenced by his ability to use the Spirit Bomb.
Zamasu (Dragon Ball Super) was chosen to be the next Supreme Kai of Universe 10 because of his skill and pure heart. Unfortunately, his heart wasn't pure good.