Over the course of its twenty-two books, Redwall has had a few of these.
Cluny the Scourge, Big Bad of the first novel, and the template for all that follow is the series' original CM. A feared warlord, Cluny hopes to conquer all of Mossflower, and turn Redwall Abbey into his new base of operations. Ruthless and despicable, he treats his soldiers as the expendable vermin they are, conscripting otherwise peaceful rats, ferrets, stoats, and weasels into his horde, and rewarding failure with beatings, torture, or death. He shows absolutely no remorse at the deaths of his lieutenants Skullface, Shadow, Redtooth, Cheesethief, and Darkclaw, promotes dissension within the ranks, and from some of the comments he makes, is quite probably a cannibal. He takes a doormouse family hostage in order to gain access to the Abbey, fatally wounds Abbot Mortimer after taking the residents captive, and upon being outmatched by Matthias in single combat, takes the Abbey chef as a prisoner in an attempt at forcing his opponent's surrender. Regarded as a satanic figure by the superstitious, and a madbeast by everyone else, he set the standards for evil in the Redwall 'verse.
The second book, Mossflower, features QueenTsarmina Greeneyes, who begins by murdering her father and framing her good-at-heart brother for it, sentencing him to life imprisonment in the dungeons of her Castle Kotir. Tsarmina then becomes a ruthless tyrant, exacting her power over her rebel "subjects", the woodlanders, by sending a giant, mindless, and bloodcrazed rat-monster-thing to attack the woodlanders and slaughter them. Like Cluny and many other villains after her, she treats her mooks horribly, forcing her lame advisor to hobble ahead of marching troops in the hot sun after one of his plans doesn't work out. She extracts so much taxation from her kingdom that she impoverishes it, doesn't give her starving soldiers a second thought, and when provided with a means for escape from an attack by the woodlanders by her one genuinely loyal general, she leaves him to die because it will make her own getaway easier.
Mattimeo features Chickenhound, better known as the masked fox Slagar the Cruel, a true From Nobody to Nightmare psychopath, who most definitely Would Hurt a Child. And not just hurt, but brutally beat, psychologically torture, force-march for days, and sell into a nightmarish underground kingdom to be slaves for the rest of their lives. His fellow slavers don't fare much better. Slagar murders several of them himself, sells his youngest henchrat into slavery as a means of currying favour with his employer, and convinces the rest of his crew to murder one another so that he won't have to share the reward with them. Unlike most of the villains in the series he does have a Freudian Excuse, in the form of an adder bite that ruined his face and drove him around the bend, but it doesn't hold much water since even before that happened he was a thoroughly empathy-deficient teenager who did not mourn his mother's death, robbed the Abbeydwellers after they saved his life, and (admittely accidentally) killed Brother Methuselah when the old mouse caught him in the act. All the snakebite really did was remove the last of his inhibitions and cause him to think a little bigger.
Rakkety Tam gives us the very, very Ax-Crazy wolverine warlord, Gulo the Savage. He murders his father and then becomes obsessed with killing his brother so that he can get the MacGuffin. Following his brother across the sea to Mossflower, he wreaks havoc in the land, butchering anyone he comes across. And eating them, as both he and his entire horde are cannibals. His list of victims include several hares that were on their way to deliver a gift to Redwall Abbey, a tribe of river rats, and more than twenty squirrels who were doing a parade. And as a present for their friends and family, he leaves their mangled heads behind. Some of these victims were eaten alive, too. Gulo is a completely insane and sadistic brute who lives to kill and enjoys every bit of it.
Outcast of Redwall has Swartt Sixclaw, aka The Pitiless One, who began his career in monsterdom as a particularly vicious teenager. Capturing the young badger who would become Sunflash the Mace, Swartt proceeded to beat and torment him for several seasons. When Sunflash broke free and maimed Swartt's trademark sixclawed paw, Swartt vowed to have his revenge upon him. Poisoning Bowfleg, Warlord of the Eastern Flats, Swartt assumed control of his horde, and went on a seasons long rampage across the countryside, searching for Sunflash; when he encountered a band of foxes led by a vixen named Shang Damsontongue, he poisoned her as well, and assimilated her band into his own. Those who defy him die horrible deaths—he forced one rat to forcefeed a crow to his captain, choking him to death, than had the rat executed for murdering an officer; the fox, Balefur, who challenged Swartt's authority was left to die in an adders' den alongside two of his closest friends. Even the death of the vixen, Nightshade, his closest advisor and long-time companion fails to get any reaction out of Swartt. And that's without getting into his treatment of his family. Swartt essentially ignores the death of his wife, Bluefen, and abandons his son Veil by the side of the road when the going gets tough. During the finale he and Veil are reunited, and Swartt proceeds to belittle and mock his son, before killing him while trying to murder one of his friends. Sadistic, treacherous, and driven solely by the need to assauge his wounded pride, Swartt is about as selfish as even a ferret can get.
Lord Brocktree gives us Tsarmina's uncle, the truly vile Ungatt Trunn. Trunn believes the path of conquest is his destiny and wholeheartedly embraces it with no regard for friend, foe or family alike, abandoning his homeland because inheriting the throne from his ailing father would wound his considerable ego. Trunn is notable for his so far unique supremacist philosophy as well. His Blue Hordes are considered the 'master race' and members of the "lesser orders" are to be enslaved or wiped out. He conquers Salamandastron, resulting in the deaths of many heroes, including its old badger lord, and brutally treats the survivors as slaves. His own men are terrified of him, and his personal seer only works for him because Trunn slaughtered his family long ago, something Trunn mocks him over again and again. One of the worst things Trunn does is his decision when supplies run low in the conquered mountain. He simply proposes eating the hostages they have. It's worth noting that despite his claims of superiority in everything, Trunn is wholly without honor. After being challenged to a one on one duel by Lord Brocktree, Trunn accepts, then instructs his men to murder the Badger Lord if Trunn loses the advantage.