Conda Challis. During his appearance as the notional villain of Orphan Star, he displays a truly legendary capacity for depravity. Among other things, he attempts to recruit Flinx to create memory-plays in the Janus Jewels, and the "scenes" he wants to see turn the stomach of even a hardened Street Urchin like Flinx. He loves torture as well, but the best example is what he plans to do with his "adopted daughter", Mahnahmi — raise her to maturity and then use her as a sex slave. It turns out that he was being telepathically manipulated by her much of the time, simply to keep him functional as a human being rather than descending into a vile pit of vice, which triggers his Villainous Breakdown when he discovers the fact.
Lord Dominic Estes Rose, a major villain in Bloodhype, is a drug dealer who trades in substances other dealers won't even touch, like the titular bloodhype. Like Challis, he enjoys torture, and responds to news of the escape of a set of captives by crushing some songbirds in a cage. He then attempts to blackmail the AAnn on Repler by threatening to release his stash of bloodhype in their compound, horribly killing all of them, and manages to provoke the AAnn commander into declaring him the most disgusting being he's ever seen. His hubris reaches its pinnacle when he attempts to sell humanity out to the Vom in exchange for his survival, and promptly dies.
Tonx Ghin Rakossa, Landgrave of Poyolavomaar in Mission To Moulokin, is an insane sadist implied to have murdered his brother for the throne. Obsessed with the capture and abuse of his slave-concubine Teeliam, he launches an invasion of peaceful Moulokin that ends in disaster for his fleet. Abandoned on the open ice of Tran-Ky-Ky by his disgusted surviving troops and wounded by a co-conspirator he'd similarly deserted, he's found by Teeliam, who pays him back in full for years of torture.
Inferred Holocaust: Better hope nobody lives in the universe which Flinx dumped the Great Evil into at the end of Flinx Transcendent. Also, once the Great Evil has been dealt with, what will happen if the Xunca decide it's safe to return to their original galaxy and kick out the squatters?