Being in the dark is stressful. Vrex haven't got enough sleep these past few days. At a young age, he needs to rule the whole realm. His Father, the Demon King was locked up after he was born. He was defeated by Meliodas, the previous King of the Dexters.
He promised to himself and to his Father that he will protect and avenge the realm. His Father told him about the Legend and the Dexters are the reason why they need to suffer in a hell-like place. They were once a normal villagers and farmers of Celestium, not until their peaceful land suddenly became a miserable hell.
Green and healthy lands were burned. Unkillable fire spreaded everywhere. They tried to get help from the King but they were attacked and King Raymond made a barrier between the realms. Brice offered his help and the people of the Sinisters Realm was given a chance to live with the hellfire.