Surprised this guy hasn't been brought up before.
What's the work?
Teen Titans was an animated series that followed five teenage superheroes defending their city (named Jump City in the spinoff comics and episode transcripts). It was an adaptation of the DC Teen Titans comics.
Who is He/What Has He Done
Brother Blood (Teen Titans) is the arc villain of season 3. As the Series had seasons that each put one of the Titans in the spotlight (with exception of Starfire due to Beast Boy having two different seasons and the show being canceled after season 5), BB acts as Cyborg's personal foe. He is the new headmaster of the HIVE Academy, a school for villains. He is a powerful telepath with many abilities.
In his first episode, he sent his students to steal things for him to help him build a device that could enhance the power of any energy weapon. Cyborg goes undercover to investigate, and when Blood finds out, he tries to use his mental powers to make Cyborg work for him under the false pretense of being able to restore Cyborg's human body (something he cannot do). He tries to place Cyborg under mind control and order him to kill the other Titans, his own friends/teammates. Cyborg however was faking it, and uses the amplifier to enhance his sonic cannon, destroying the academy. Blood vows revenge for this.
In his second episode, Blood is revealed to have built a device designed to create tidal waves large enough to completely wipe out Jump City. When the Titans try to stop him, he of course tries to kill them. He is defeated however and his base is destroyed.
He last appears in the two part season 3 finale. In it, he breaks into Titans East tower and places all of Titans East minus Cyborg under mind control. He also has Professor Chang transform him into a cyborg as well. He reveals that he plans to surgically operate on the Titans East (who I remind you are teenagers) and convert them into cyborgs as well. He openly admits that not all of them will survive the process and that once it is complete, they will be unable to ever break free from his mind control ever again and will be his cyborg slaves forever (at this point I would like to point out that it is indicated that those under his mind control are still aware of what is going on but can't do anything about it), furthermore, he also reveals that after he finishes with them, he plans to kidnap thousands of people to convert into his cyborg slaves, stating he will have an army of thousands of them. He orders the brainwashed heroes to kill Cyborg by dismantiling his body while he is strapped to an operating table so he can find the thing in his implants that allow him to resist Blood's mind control, not wanting to have the same problem with his army. When the core Titans stop the brainwashed Titans East from doing this, Blood whisks the brainwashed heroes away, stating he will just have to go through trial and error with them, noting that if any of them do rebel, they will be disposed of. When the Core Titans arrive and stop him from doing this, he leaves the brainwashed TE to fight the Core Titans while he deals with Cyborg. Blood than proceeds to give Cyborg a brutal beating, tearing apart his robotic body and leaving him a limbless mess on the verge of death. Blood eventually uses his powers to teleport the TE and Core Titans to their location so they can be forced to watch Cyborg's death. As Blood continues to dig into Cyborg's circuitry, Cyborg eventually manages to hack into Blood's, drawing energy to rebuild his robotic body and overpower Blood. Blood is defeated and sent to prison, never to be seen again.
Freudian Excuse/Mitigating Factors
None. Blood is a criminal mastermind whose only goal is to teach other villains how to be evil, and he is willing to destroy lives to do this. He also has no real care for his students, threatening to have Jinx, Gizmo, and Mammoth killed for failure and keeping all of his students under mind control to force them to cheer for him.
This is where things get tricky. The series has a very high heinous standard. Slade tried to burn down a village, injected devices into the Titans to force Robin to obey him under threat of slowly and painfully killing his friends, tried to force Terra to kill Beast Boy, left behind a mask filled with a substance that caused Robin to hallucinate that Slade was still there and which resulted in Robin nearly being beaten to death, tried to help Trigon destroy the world in exchange for being brought back to life, and sent one of his android duplicates to attack Beast Boy and mock him about Terra. Trigon tried to destroy the world and conquer the universe. The Brain attempted to use a quantum generator to wipe out entire cities, horribly tortured the Doom Patrol, and tried to detonate his base to wipe out all the heroes and his own allies while he escaped, noting that the explosion would also take out a large section of Paris. Even minor villain Professor Chang tried to wipe out a large section of Jump City with a disintergrator cannon and performed horrible experiments on Red Star.
All of that said though, Blood still does a lot. He tries to wipe out an entire city and was planning to convert innocent teenagers and then thousands of civilians into his cyborg slaves, a process that would be incredibly painful, not all of them would survive, and would leave them fully aware of what was going on but unable to do anything about it. Just the fact he was going to operate on young teenagers and convert them into his cyborg slaves before spreading to the rest of the city is messed up in so many levels. He also has a nasty habit of making brainwashed heroes attack and try to kill their own allies. He tried to make Cyborg destroy the other Titans and ordered the brainwashed Titans East to kill Cyborg by slowly tearing him apart while he was strapped to an operating table. Finally, he delivers an especially vicious beating to Cyborg while forcing his friends to watch.
Now, right off the bat, Trigon is an interdimensional demon, so let's not even compare Blood to him. Blood should be judged by how he measures up against other villains of his tier, meaning Slade before he worked for Trigon and got much more power, Brain, and Chang. Now, Slade tried to burn down a small village, which is pretty low on population compared to a city. Blood also has the attempted transforming of thousands of innocent people into his cyborg slaves, which I believe is far worse than anything Slade did without Trigon's help. Chang's experiments on Red Star were certainly horrible, but the same goes for operating on people and turning them into cyborg slaves, and while Chang only did it to one person, Blood was going to do it to thousands. Brain is probably his biggest competition, what with his wanting a device that could wipe out cities and trying to destroy a large section of Paris. Still, Blood has the addition of personal sadistic cruelty by forcing brainwashed heroes to attack and kill their own friends and delivering a horrible beating to Cyborg while making his friends watch. I would also like to argue that the fact that the thousands of people he'd convert into his cyborg slaves would all be aware of what was happening but wouldn't be able to do anything about it and would be stuck like that for the rest of their lives actually makes the planned action much worse than wiping people out like the other villains attempted due to it being a fate worse than death.
Moral Event Horizon
If he didn't cross it by attempting to mind control Cyborg into killing his own team, he certainly did when he tried to wipe out all of Jump City with a tsunami.
Despite the high heinous standard, I believe he counts. He stands out from the other villains due to his And I Must Scream mind control and plan to inflict a fate worse than death on thousands of innocent people. Even Slade couldn't out heinous him without the aid of Trigon. A few villains try to destroy cities, but he has that plus his horrible plan to convert people into cyborgs.