These guys aren't villains in the classic sense, but they are men driven by their own ambitions and agendas. One sees himself as the savior of the Federation, one the savior of colonists wronged by the Federation, and one striving for his own glory above that of the Empire.
Luther Sloan is a member of a secretive organization, one so secret that even the Federation can't, or won't, acknowledge. It's Section 31 and it's soldier pays DS9 a visit, seeking out traitors. In "Inquisition," he grills Dr. Bashir about his time spent in captivity by the Founders and whether or not his loyalties have turned.
But his ulterior motive was to get Bashir working for Section 31 as a spy. One of the reasons, we learn in "Inter Arma Enim Silent Leges," was to develop a bio weapon to use against a Romulan candidate for the Senate. He seals his own fate in "Extreme Measures" by committing suicide when Bashir turns the tables and entraps him to reveal a cure for a plague Section 31 created to destroy the Changelings.
Michael Eddington was a security officer stationed on DS9 after it was revealed the Founders were like Odo---changelings. Eddington and Odo learned to trust each other in the beginning, but later we find Eddington was secretly working for the Maquis. In "For the Cause," intercepted replicators enroute to the Dominion are then stolen by him and Maquis sympathizers.
Sisko hunts down Eddington after a long game of cat and mouse. In "For the Uniform," Eddington, after having attacked several Cardassian colonies in an act of war, finally surrenders to Sisko and the Federation after Sisko tells him he's about to launch bio weapons torpedoes on Maquis colonists.
In "A Blaze of Glory," Eddington meets his fate when he and Sisko are forced to stop cloaked missiles enroute to Cardassia being launched by the Maquis. They end up at an outpost where Eddington dies heroically, battling an attacking troop of Jem'Hadar.
In "The Way of the Warrior," Gowron is head of the Klingon High Council and he orders crews to invade Cardassian colonies and take them over in an act of war. Before Cardassia became allies with the Dominion under the dictatorship of Gul Dukat, Gowron is suspicious that members of Starfleet headquarters and the Cardassian civilian leadership have been infiltrated and taken over by Changelings.
He gets pissed when Worf refuses to join him on his quest to invade Cardassia and strips Worf of his title and a place in the Empire. He tells Worf he will have nothing to show for turning his back on him (Gowron, not the Empire) and Worf tells him "except my honor."
Later, the Empire reluctantly rejoins the Federation when Cardassia becomes a friend to, and under the protection of, the Dominion. But that doesn't stop this Klingon egomaniac from trying to gain complete control, not just for self indulgence, but out of spite and jealousy of his rival Martok. His fate is sealed when Worf calls him on his unwise and reckless decisions that had cost Kronos several thousand men and ships in unwinnable invasions. In "Tacking to the Wind," they engage in a duel with Worf coming out the victor and Martok taking the mantle of Leader of the High Council.