Lang from the third game is one of the earliest examples. It's mentioned in the first game that he was a noble of Archaneia who betrayed his nation to Medeus and stole wealth from the poor, then went back to Archaneia once they start winning again. He is then made overseer of the defeated Grust, replacing Lorenz. It's mentioned, in the most kid-friendly way possible, that he steals wealth from the poor, kills boys over ten, rapes and kills girls over ten, and kills entire villages if even one person rebels (a technique the Schwarze Rosen will later pick up). When you attack him, Lang says that he was forced to do this by his boss, but it is simply a trick to get the first attack in. Even Medeus and Gharnef have vaguely defined backstories on why they're bad; Lang is simply pure evil with no explanation. This guy is so over the top that he exists to make Marth realize that there's something wrong with Archaneia and to join the rebellion. It's vaguely implied that his actions were done to provoke Marth into conflict, making him look like a traitor.
Gharnef still qualifies, though. He turns evil and later killed Linde's dad, Miloah, solely because their master gave him the Aura spellbook, led the attack on Altea that killed King Cornelius, stealing the Falchion and kidnapping Elice. He brainwashed Tiki into attacking Marth and manipulated Hardin, corrupting him via the Darksphere. He also kidnaps four innocent girls (Elice, Nyna, Rena, and Maria) as sacrifices for Medeus's resurrection. Worse, this also causes a huge war that turns the continent to hell and ruins the lives of many.
The Archanea remakes only gave us more reason to despise him — in FE 12, he brainwashed the kind-hearted caretaker of a destroyed orphanage, Eremiya, to make her the horrible Tyke Bomb training bitch we met and fought. And once she lost against Marth's group, he restored her memories... purely to mock her as she died in despair. When you engage in such pettiness as warping a broken person's personality like that — probably knowing full-well that they'll fail — just to relish in their anguish, you almost definitely deserve a place in this category.
In Genealogy of Holy War, we have Queen Hilda, the power-hungry queen of Freege. As you gather information from village to village, you learn that she wholeheartedly supports the movement of sacrificing kids to an evil god and did it For the Evulz. Later on, it is revealed that she is responsible for the death of one of the first generation characters, Tiltyu, by abusing her and her daughter Tinny (a 2nd generation character), again For the Evulz (using 'being a traitor' as an "excuse"), the former to death. Hilda takes extreme pride in her actions, gloating about it in front of her children. Hilda is just as much an abusive bitch if Tiltyu dies child-less. In that case, her and Blume's little sister Ethnia will replace her as Hilda's punching bags, alongside her children, Amid and Linda (replacements for Arthur and Tinny). And although her daughter, Dark Magical Girl Ishtar, was not physically abused, she was still manipulated by her Evil Matriarch mom because of her love for Yurius, which Hilda saw as a chance to gain even more power than she already had.
She's even recognized as such In-Universe — the villagers you visit talking about her spoke on just how utterly cruel she was and consider her a soulless monster.
That Queen Hilda of Chronos is wicked to the core. She began abducting kids after allying with this dark bishop. She kills the mothers, too, without a blink of an eye! That woman's not even human. I'd say she's more like a devil!
First Generation: He manipulated various rulers and dukes, causing the war against Isaac by using Duke Reptor of Freege to assassinate Ira's father, King Mananan, while also manipulating King Chagall into killing his father and sending his envoy, Sandima, to stir up trouble in Verdane. During this time, he worked primarily with Alvis, whom he was blackmailing with the knowledge of Alvis's Loputo blood, in addition to his Fala heritage, thus poisoning him with hatred and envy. Later, he kidnaps and brainwashes Sigurd's wife, Dierdre (and in one of the mangas, he outright mind rapes her and laughs about it), then brainwashes her and the emotionally-weakened Alvis so they'd marry and have children who could be used as Loputo vessels. And as the cherry on top, he kills Levin of Silesia with his own hands in the Battle of Barhara (with Holsety becoming a Dragon Ex Machina to let him escape death).
Second Generation: Between the first and second generations, he gives Prince Yurius the Loputo tome, causing him to become the vessel for the Dark God. As such, Yurius kills his mother Dierdre and attempts to kill his twin sister Yuria, shaping him into the horrible rival seen later. Manfloy also attacked his pregnant daughter and his son-in-law purely because they broke his Parental Marriage Veto, killing him and driving her into insanity, causing his soon-to-be granddaughter Sara to be pissed off at him forever. And after all of that? He organized and helped commit the horrible child hunts already mentioned, and later kidnapped Yuria (who had been taken in by Levin, and later by Celice's army) and brainwashed her so that she'd become his and Yurius's Apocalypse Maiden.
Izuka, Ashnard's demented chief scholar, is no less evil than his master. In his brief cameo in Path of Radiance, he appears onscreen just long enough for it to be revealed that he is the developer of the Feral Ones, Laguz who are forcibly mode-locked into their beast forms and driven insane to be used as cannon fodder. In his base at Gritnea Tower it is revealed he has an entire basement devoted to storing the corpses of his experiments' victims. Izuka takes on a much greater role in the second game, where he has been hired by Lekain to bend Daein to Begnion's whims. He takes on the role of strategist to Daein's weak-willed Prince Pelleas, where it is demonstrated that he would gladly sacrifice good strategy for pointless destruction. He injects the innocent Warrior Poet Muarim with the Feral One drug, which would have annihilated his mind if not for Rafiel's intervention. When Begnion offers to make peace with Daein, Izuka springs his trap, telling Pelleas to sign a "peace treaty" that is actually a Blood Pact, mystically binding Pelleas and thus all of Daein into virtual slavery to Begnion. Finally, it is revealed that he's tested his drug on the Beorc, too, including turning Elincia's noble Uncle Renning into the Tragic Monster known as General Bertram.
Ashnard. He's not called the "Mad King" for nothing. The pinnacle of a Social Darwinist, he became the King of Daein by getting his father to sign a blood pact, and then invoking it, killing everyone else who was in his way to the throne, and countless other innocent people. He then proceeded to kill his own father, therefore becoming king. After finding out about the dark god (really the goddess of chaos, Yune) sealed in Lehran's Medallion, he decides to release it for the lulz. This requires a war that spans the whole continent, so he decides to begin by invading the next country over. He also, after finding out that his own son Soren was unable to transform into a dragon like the child's mother Almedha used to be able to, decides to hold him captive to get ahold of the kid's uncle, the oldest son of the king of Goldoa. He then proceeds to warp Rajaion's body and turn him into something resembling a wyvern, and then uses him as a mount. He laughs at everything. His ideal world, the one that he wishes to create, is one where the only thing that matters is power. And to top it off, his chief court scientist is none other than Izuka himself.