Dio Brando from JoJo's Bizarre Adventure Part 1: Phantom Blood.
Warning: Wall o' text and spoilers ahead!
At the age of 12 Dio was adopted by the wealthy George Joestar after Dio killed his own father. Upon arrival to this family Dio immediately took advantage of this kindness and started making life hell for Jonathan, the only son of George, intent on making Jonathan commit suicide from depression so that once George died (Dio was slowly killing George by slipping poison into his drinks) Dio would inherit the Joestar fortune. Dio beat the shit out of Jonathan, stole Jonathan's girlfriend's first kiss, publicly humiliated him, made him look bad in the eyes of his father, stole his friends, and shoved his dog Danny into an incinerator. Dio was still 12 at this time.
After discovering an ancient artifact and being exposed for his attempted murder of George, Dio would use the artifact to become a bloodthirsty vampire. Dio would kill George, but be seriously wounded from his fight with Jonathan right afterwards. As a vampire, Dio needed blood to recover. A lot of it. Next time we see him fully recovered, Dio had already drained an entire city and turned the population into his zombie slaves. Upon being asked how much blood he's drained Dio simply responds by saying "Do you remember how much bread you've ever eaten?" Jonathan, his friends, and his master Zeppelli take it upon themselves to stop Dio, but Dio's zombies kill Zeppelli and Dio himself kills one of Jonathan's comrades.
Dio's actions as a vampire also include making terrible monsters that are the result of taking zombie parts and mixing them with various animals and
making a mother eat her own baby.
Eventually, Dio is defeated by Jonathan, seemingly for good. So Jonathan gets married and plans to take a boat to America to spend his honeymoon. Little did he know that Dio won't stay dead, and is on the ship. However he's so badly wounded that he's just a decapitated head at this point, and needs a strong new body. Who better than his brother Jonathan? Dio catches Jonathan by surprise and kills him right in front of his wife, takes his body, and rests for 100 years to return in Part 3.
Honestly, Dio is a monster, but I think I hate him the most for his last act in Part 1. Throughout Phantom Blood, we see Jonathan grow from a meek little boy into a real man. He made countless sacrifices and never once strayed from what he thought was right. He went after Dio because he felt obligated to cleanse Dio's evil from the world to avenge his father, and eventually his master. Jonathan, by all rights, deserved a happy ending, but it was cut off by Dio. Even in his last moments he saved his wife and a baby (the only two to survive on the ship) and even
forgave Dio. Saying that he still felt a strange sense of brotherhood as his life ended.
However, this is just Part 1. This doesn't account for the consequences of Dio's actions in Part 2, or his return in Part 3, or how his actions throughout the years eventually kill every protagonist from parts 2-6 (kinda).
