Darkonda. He was the one who kidnapped Karone in the first place and set her up to become Astronema, and on a few occasions, he forced Ecliptor to merge with him against his will. He even has no qualms about stabbing his friends in the back, as he was the one to kill Dark Specter—-all for trying to take over his position as Big Bad, and suffers a Karmic Death as a result.
Deviot. In a show full of villains with standards, Deviot had none. He was a manipulative, treacherous Starscream who would cheerfully throwaway his men's lives, thought brainwashing was a useful tool, and had no loyalty to anyone. Deviot destroyed Scorpius to get his hands on the cocoon that had been intended for Trakeena to awaken her true potential. He hurtles the Terra Venture into another dimension, and finally fights Trakeena for the cocoon, until they both fall in. His influence turned Trakeena into a Complete Monster too. Under said influence, she organized suicide bombers, killed her mentor for refusing to fire on civilians, and later tried to drain the lifeforce of innocent people to power herself up. In other words, it was him inside Trakeena's body and in full control.
Queen Bansheera; even in shows for adults, you'd be hard-pressed to find a worse boss. She has five main minions, and she's responsible for the deaths of four of them, and almost kills the last one, draining the life from one, blowing another to smithereens simply for being in the way, erasing the minds of the last two, Diabolico and Olympius (Her own son), forcing the Rangers to kill them, leaving only one, who remembered right before Bansheera killed him that he prepared an attack on the Aquabase, sparing his life. She's the only Power Rangers villain to get a Fate Worse than Death, as she ends up falling into the local Hell equivalent to be tortured for all eternity (or at least had her soul ripped out by demons. Oh, and the one who finally defeats her? The demon who performed a Heel-Face Turn, Diabolico, the only one who seemed to realize Bansheera was pure evil. Diabolico was the Big Bad at the beginning of the season. Bansheera was just a Bigger Bad at this point. She was that evil.
When Master Xandred isn't drinking his "medicine" and yelling at people, he shows how evil he is, being the Big Bad and all. He treats his followers horribly, yelling at them, threatening to kill Octoroo at one point, attempts to kill Deker because he was meddling in his affairs, and his plan revolves around killing Jayden. Unlike Octoroo, who sends Nighloks to cause misery, Master Xandred summons an army of moogers to cause meaningless chaos in the human world and destroy everything. In episodes "Broken Dreams" and "Test of the Leader", we see that his moogers destroy entire buildings, meaning that he is responsible for hundreds of deaths of innocent humans. He brutally punishes Arachnitor when he tried to usurp him by mutating him into a mindless berserker slave to point at the Rangers and walk away. Granted Arachnitor was a traitor, but still. Even Octoroo says that his cruelty is boundless. In "Broken Dreams", he ruined Dayu's harmonium and attempted to erase her memory. She even says "I knew you were evil, but this..."
Eventually, after Deker meets his Final Death and Dayu's sorrow is so great that she is able to fully restore Xandred on her own, he rewards her by absorbing her so that her human side would make him immune to the sealing symbol. However, by this point, Dayu didn't care if she lived or died, and didn't resist.