Star Trek: 10 Underrated Villains
7. Luther Sloan
The Romulans and the Cardassians were considered to be two of the worst empires in the Alpha Quadrant. They could be ambitious and ruthless in the worst possible ways. Both of them had clandestine organizations that worked to to protect them from their own internal threats, respectively the Tal Shiar and the Obsidian Order.
Surprisingly, the Federation had a similar organization known as Section 31. Well, it would have been if anyone knew about it. Section 31 had reputedly existed since the earliest days of the Federation, with allowances for it in the Federation's Charter. But over the centuries, Section 31 became a secretive and dangerous organization with zero oversight, taking out threats to the Federation both real and perceived.
Virtually no one in Starfleet even knew they existed until Luther Sloan showed up on Deep Space Nine. His goal was to recruit Julian Bashir, though Bashir played that against him over time to collect information on Section 31. Sloan was a different kind of villain as he showed the darker side of the Federation, one that surprised a lot of fans.
Section 31 also showed up in Enterprise and Discovery, though Luther Sloan was their greatest, and possibly most evil, agent.