The Beast is a very complex character.
He goes through so many changes in his life and to me they are very belivable and here I will discuss why. First his childhood:
Not much is know of his life before the enchantment except that he was like the prolouge says "Spoiled, Selfish and unkind" but that is all I need, I belive that he got the position of master at an early age and that made him that way.
I quess his parents died sometime before the story and that gave him too much power for a child and it gave him the character flaws that the Enchantress cursed him for.Next his Teen years: