Akainu

"We have to be thorough! If even one of those scholars made it onto that ship, all of the sacrifices we've made would be for nothing! We must eliminate the very possibility of this "evil" existing!!"

- Akainu after destroying a refugee ship

Akainu (real name: Sakazuki) is one the 3 Marine Admirals in the anime/manga One Piece and he's one of the most powerful fighters the Marines have to offer. He ate the Magma-Magma Fruit making him become living magma. He's cruel and heartless, and believes in the Marines motto of "Absolute Justice", which is to punish criminals at any cost. He will kill anyone that he thinks are evil or are an obstacle of justice. He will also not hesitate kill his own comrades who run away from battle he will kill them no matter how they beg for mercy.

His debut in the series features him immediately crossing the Moral Event Horizon by destroying a ship filled with innocent refugees on the miniscule possibility that a criminal might have been on board, an action that horrified even his comrades. Since then, he rose through the ranks of the Marines and eventually became one of the Admirals, alongside Aokiji (who detested him for his cruelty and fanaticism), and Kizaru.

He's the one who killed Monkey D. Luffy's brother, Portgas D. Ace. His magma overpowered Ace's fire power, and punched him through the stomach as Ace was trying to protect Luffy. He even tried to kill Coby when he said further fighting would be pointless, but luckily, Shanks saved him just in time. Through this horrble action, Akainu accomplished what no other villain ever could: completely breaking Luffy's spirit.

Within the two year time-skip, Akainu officially became the new Fleet Admiral, officially placing him as the commander of all the World Government's forces, which was later revealed to the Straw Hats by Jinbe at Fishman Island.