Note the order below is by when they appear, not by ranking whatsoever!
Mason from Bioman- I have to say Mason seems to be more cruel than his master Dr. Man himself. In episode 10, he took pleasure in killing Mika and if that's not all, he laughed as he saw her die. In episode 12, he thought of crushing Shingo's emotions by making him doubt himself. I think his worst has to be in episodes 43-44 where he continued to torture Miki the mechaclone who was connected to Satan Megas' invincibility. He also took delight at the Biomen's attachment to torture them further.
Zimba- His first apparently moral horizon event was in episode 21 as all the fun turned to gloom when he severed the friendship between Daichi and Sumo Bouma out of cold blood. He then ordered Zulten to turn Sumo Bouma into his violent form so he could watch the Turborangers suffer. Or perhaps in episode 24 where he manipulated a poor dog that Haruna saves so he can get Haruna and turn her into one.
Jarmin from Turboranger- Arguably, she's displayed the most cruelty in the show for some. Her first moral horizon event was when she tricked the two brothers Fire and Ice to merge into one by making them fight with each other in episode 19. Or another in episode 27, she tortured Chime Bouma in a very unspeakable way which she actually destroyed her people for not serving her and whipped her severely many times. It was arguably the most cruel act in Turboranger no wonder Zimba is impressed with her.
Zeba from Maskman- In the final episode, he is revealed to have little or mercy as he double-crossed his own subordinates to get what he wants.
Radiguet from Jetman- Yep from the worst bastard in Super Sentai, he doesn't give a damn to what he does. His first could be when he actually killed a girl he grew to care for even after she took care of him during his amnesia. If that's not enough, another I would count was when instead of killing Lou from Berserk, he actually provoked her anger many times just to make sure she becomes his but when that failed, he decided to use her fiance Duran against her to show he's a sore loser. Perhaps his greatest moral event horizon has to be Maria because as he grew to want her, he only showed desire and not real love. When he lost her, he killed her not just to satisfy his bloody desires or revenge for unrequited love, but also to make Ryu's mission to save her a failure.
Ryuon from Boukenger- No doubt the most cruel Boukenger villain ever. In Tasks 15-16 he promised Ragi that he will help revive the water Metropolis but at the condition he got rid of the insignia, turned Ragi into a dragon and when the Aqua Crystal was not found, he brutally beat him. In Tasks 33-34 he together with the Questers turned Natsuki against the Boukengers and planned to use her life force to power his greatest Jaryuu robot.
Long- This one is another so cruel I guess he's got a lot. He has manipulated Jan's dad Dan, he also double-crossed Rio and Mele for his personal purposes just to prove how sick he really can get. The worst part is where he kept lying to them, speaking so softly and gently to them as if he were kindly but he was in reality full of evil. I guess that's why some fans agree he is worse than Radiguet.
Any more to add? Feel free to COMMENT your own!