Back Dimensional Count Radiguet is a sadisticAx-Crazy genocidal maniac with megalomaniacal fantasies of ruling the Vyram under an iron-fisted dictatorship. Even when Juza, his boss and the true Big Bad, arrived he planned to overthrow her but that failed. Afterwards when it looked like he would become good because of a sick girl (who became his Morality Pet while he had amnesia) whom he cured unknowingly he spurned her offer to turn good by killing her. All because she believed he could feel love. He's also the one who brainwashed Rie into being Maria for no other reason For the Evulz. His most repulsive Kick the Dog moment was when he killed her after she snapped back to being Rie. There's also the part where he tortured Tranza until he became insane and was left at the door of an asylum.
Commander Guirail is a seriously depraved creature with love of underhanded tactics, whatever it takes to win. Even children wouldn't be spared. His favorite tactic seems to be using human shields. Anyone's fair game, even his ally Yugande: After "tricking" him to work together for the common cause, Guirail makes him take full brunt of damage when the chips are down against him. He's the only one amongst Nejirejia Kingdom who knows nothing of "family/companionship" value: The moment Yugande was subjected of those things, both Hinelar and Shibolena were thoroughly disgusted and socked him with a Psycho Serum under the guise of power-up in revenge of what he did to Yugande.
Grand WitchGrandienne deliberately sent her children out to die, brainwashed one to kill another on several occasions, banished one to Hell, and resurrected two of them as brainwashed zombies. As fitting of the family theme in GoGoFive, Grandienne stands for everything the Tatsumi family bond isn't.
Captain Ryuya, Tatsuya Asami's Identical Grandson (give or take a few dozen "greats"), was the ultimate mastermind behind everything that occurred in the series, from the initial escape of Don Dolneiro to Gien's insanity; he was responsible for the crisis that took place in the finale (he had seen two futures — one in which 1/3 of the 21st century is destroyed and one in which the 31st century ceases to exist). To make matters worse, the only reason he was trying to manipulate everything was that he had received a vision that he would die in both futures.
Agent Abrella sold many weapons supporting galaxy-destroying wars for fun and profit to most of the Alienizers, regardless of how many lives were taken. Once a galaxy is destroyed, he moves to the next to generate profit and destruction. Abrella treats any and all crime as a business opportunity, and it doesn't matter if you're destroying cities, selling slaves, pawning stolen merchandise, or filming child pornography, as long as the price is right, he'll be more than happy to sell you the tools you need to further your enterprise. And his Villainous Breakdown near the end of the series was caused merely because the Dekarangers were ruining his transactions. Yes, Abrella himself caused a citywide destruction spree and nearly destroyed the whole of the Space Police ''just so he could keep his job''.
Long the Infinity Dragon has to remove all worldly attachments from a person by systematically destroying everything they love in order to create the god of destruction and destroy the world. He did it four thousand years ago to Maku, causing the original rebellion, the Kensei had to use a forbidden technique and be turned into animals before he could destroy the world. Years later, Long destroys Jan and Rio's hometown. Jan loses his mother, forcing him to be raised in the jungle. Rio is taken in by Master Shafu, but eventually turns away and seeks evil power to get revenge on anyone. His motivation is boredom, and since he's an ancient being, he thinks he has all the rights to manipulate these lesser creatures for his own amusement at their expense.
If it's possible for a show that's relatively dark from the beginning to have a Knight of Cerebus, Sujigarano Akumaro would qualify. The Gedoushu at large wanted to create enough anguish to cause the Sanzu River to rise high enough to flood the world of the living, thus enabling their incursion thereof. Akumaro, on the other hand, wanted to open a portal to Hell because he wanted to enjoy absolute human suffering — even though said opening would destroy both the human world and the Sanzu River. He also made Fuwa Juzo's sword from Juzo's dead parents, whom he murdered himself. His monstrosity extends to even the Ayakashi who serve under him; they seem specifically made to cause as much suffering as possible in order to further his goal (using said suffering as a wedge into one of six precise points along a line, at the center of which was a portal to Hell).
Fuwa Juzo was a remorseless Blood Knight to the point that even his fellow Gedoushu didn't trust him; he even only became a Gedoushu in the first place so that he could keep killing in spite of his sickness. Akumaro saw potential in him for his ability to walk the line between humanity and the Gedoushu and sought to use him to open the gates of Hell. However, he throws a sharp wrench into the plan, declaring that he already knew that his sword made by Akumaro was made of his parents (a fact which Akumaro had recently told him to bring him to a Despair Event Horizon), and that he didn't care.