I watched King Arthur: Legend of the Sword (2017) and one character got to me; VORTIGERN; Brother to Uther Pendragon and Uncle to Arthur, powerful warlock and tyrannical ruler of Camelot.

 Jealous of his brother’s reign as King, Vortigern conspired with Mordred to have him kill Uther during his invasion against Camelot to establish the mages reign over humanity. They fail, Vortigern realizes he needs more power to defeat Uther who had Excalibur, the sword.
Vortigern sacrificed his wife, Elsa to sea witches in exchange for dark powers as part of preparation of his coup against Uther. He transforms into a demonic scythe knight and defeats Uther but Arthur who was a child escapes.

Vortigern becomes king and kills all the sorcerers so no one could rival his power. 
Years later, young Arthur grows into a strong and skilled warrior and comes to claim his throne. As Arthur leads a revolution with the people of Camelot. A desperate Vortigern afraid of losing his power goes back to the sea witches. They require another sacrifice, this time, his daughter. Vortigen remorsefully sacrifices his only daughter and gets his demon power back. 

After a heated battle, Vortigern is defeated. 

Though he actually loved his wife and his daughter to a point, his love for them was ultimately outweighed by his lust for power, and so in the end he was willing to sacrifice them to secure his rule over Camelot.

TEMPTATION BLACKENS THE HEART
Giving in to temptation has a corrupting influence, slowly darkening one’s heart and leading them down a path of moral compromise.
“Guard your heart....” Prov. 4:23