Several of the Celestial Dragons are slave-owning arrogant bastards. When a slave tries to escape and blows up his collar, St. Charlia of the Celestial Dragons shoots him while complaining about how pathetic it was that he was whining about his family (who he just wanted to see again). Her brother, St. Charloss (introduced while riding on a slave), enraged that a doctor and a nurse carrying a wounded man would dare not to kneel in front of him, kicks the injured man off of the stretcher. He then decides to take the nurse as his wife. When her fiancee complains, he shoots him.
Shiliew/Shiryu of the Rain, former Head Jailer of Impel Down counts. Completely Ax-Crazy, he murders dozens, if not hundreds of prisoners for fun, claiming them to be "just trash" and making the actual Head Jailer Magellan completely disgusted. When freed, he cuts down his former workmates multiple times with not regard for them and sides with Blackbeard. Also, he thinks of betraying Blackbeard if he does not succeed in stealing Whitebeard's power. It's pretty clear he has no empathy and loyalty for anyone but himself.
Admiral Akainu, a wonderful example of a Knight Templar who crosses the barrier between Well-Intentioned Extremist and Complete Monster. He debuts while crossing the Moral Event Horizon: he orders his subordinates to blow up a ship of evacuees that the Marines had promised to spare. His justification was that he thought that if there were any survivors from Ohara that the world would be in grave danger, although there's no proof that survivors of Ohara actually present any danger. Later in life, he brutally murders anyone who tries to flee from the battle at Marineford. Then there's what he does to Coby. The kid is pretty much the Only Sane Man and calls out everyone for their violence. Akainu immediately tries to kill him - and they're theoretically on the same side. If not for Shanks...He moves the series into Darker and Edgier territory by causing the first major present-day story death. Let's put it this way: when Sengoku stepped down and Aokiji had to fight Akainu for the position, which Aokiji ultimately lost, he left the Marines rather than serve under him. The guy is that bad.
Hody Jones, a Fishman who takes Arlong's Fantastic Racism and turns it Up to Eleven. He's more brutal, more racist, more treacherous, and more cruel, but the kicker? He pulls a reversal on Arlong's sole Pet the Dog moment; whereas Arlong was mortified when Luffy used a Fishman as a human shield, Hodi actually uses a Fishman as a human shield against an attack. One of Arlong's former crew, Hatchi, stated that, while Arlong was a bastard through and through, he'd never hurt a Fishman no matter what. Hody, on the other hand, is known for killing any who talk to or know them. In fact, when he tries to take over Fishman Island, part of his plan is to execute all who had signed Queen Otohime's petition. This means that he's planning to murder thousands of its residents. He already killed Otohime and framed the human he hired to burn the petitions in the first place as part of this plan... and got cocky enough to brag about it in front of all Fishman Island.
Punk Hazard gives us the truly detestable Mad Scientist Caesar Clown. A Villain with Good Publicity who's convinced people he's a kind master and scientist who just wants to help people. The truth is that Caesar is violently jealous of his former associate Dr. Vegapunk and once took that frustration out by blowing up an island. In the present, Caesar keeps multiple children in his lab with promises to cure them of their sickness, and gives them candy to help them. The 'candy' is in truth an addictive drug to keep them enslaved to him, and his experiments will cause their deaths in five years. He shows no remorse, laughing he can always get more children. Caesar also saved pirates and sailors from death, but uses them as test subjects as well and turns them against their captain who was just trying to save them from Caesar. When his back is against the wall, Caesar has no compunction killing these men with a poison gas he unleashes in an attempt to kill Luffy. Caesar has no loyalty to anyone but himself and his psychotic ego and combines scientific drive with a sadist's streak a mile long.