The Thief of Souls, known primarily by his identity Reverend Dominic Darrk, is the demonic and manipulative mastermind behind all the evil in the series, seeking to pave the way for his master, Crisis, to invade Earth. After possessing a man, banishing his wife to Dreamland, then corrupting their son into a sociopath like himself, the Thief sets up his Church of Eternal Empowerment, a cult that recruits its members through kidnapping, brainwashing, or threats of death. Along with regularly ordering assassinations and executing his own minions for the slightest of failures, the Thief unleashes monsters to rampage across entire cities more than once, and later releases a virus called the "Sleeping Death", which plunges its victims into neverending sleep while the Thief steals their souls to offer up to Crisis. In the end, even when beaten, the Thief happily taunts the heroes about the coming of his master, and the joy he will take in the knowledge that they will all spend an eternity of agony at Crisis' hands.
Crisis himself is the force behind the Thief of Souls, and the most evil being the JLA encounter. Coming to Earth after attempting to devour thousands of innocents' souls, Crisis plans to turn the entirety of the universe into his personal playground by fusing the Dreamworld with Earth, then plunging every living thing in the universe into eternal pain and suffering. After brutally tormenting the heroes with their worst fears, Crisis manipulates his descendants Robin and Melana to betray their team, at which point Crisis hopes to eternally imprison them in a gemstone while he rampages across the universe, bringing death and destruction across the cosmos simply because he can.
Armando Guitez is the Big Bad of the Wonder Woman story, and manages to stick out even among other far more powerful villains as a truly reprehensible monster. Using his wealth, Guitez begins tearing apart Mexican ruins looking for ancient and powerful runes, and threatens any nearby farmers off their land with death, ordering one beaten to death and shooting another in the head just for voicing their compaints against him. After murdering Judge Mendoza for displaying disgust at his crimes, Guitez plans to rape the man's daughter Maria before murdering her would-be rescuer, Steve Trevor. Guitez, finding the ancient runes he has been searching for, absorbs the power of the demons residing within them, travels to Los Angeles, crashes the plane he's on into a populated street, then goes on a massacre throughout the city for fun as he searches for other runes to make himself ever more powerful. Guitez's evil ran so deep, that even one of the demons he absorbed proclaimed his shock at Guitez's sheer black soul.
Harsher in Hindsight: Wayne Williams actually exists in real life, as in a man who was convicted for murder.
Hilarious in Hindsight: Batman is a black man in this series, DC also created a universe where most of their heroes are black.
Moral Event Horizon: Lady Zakara rewards Zorgul with sex, but it's just a pretext to kill him. Then when he survives it, she pretends it was an accident. It just gets worse and worse.
And Guitez, who first crosses the line by shooting Maria's father dead, and only gets worse when he plans to rape her, kills Steve Trevor, and bonds himself to the spirit of darkness to gain power for himself.
What an Idiot: Everyone in STEALTH takes this trope to an absolutely ridiculous level. Lady Zakara randomly kills her own men at the slightest provocation or failure, then tries to kill Zorguleven though he has been effective and completely loyal so far, and refuses to listen to a warning from one of her men until it's too late despite him insisting multiple times. Zorgul himself somehow buys it when she tells him his attempted murder was an accident. And the Mooks as a whole still accept to follow Zakara despite her proving several times she is the worst boss ever.