I might not be able to convince you that Slagar is the best Redwall villain- others are better armed, more dangerous, cause harm on a larger scale. Some of these guys have entire hordes at their disposal while Slagar is just one dude with a handful of cronies. In the grand scheme of things he accomplishes very little, but his scope was never large to begin with. Unlike all these warlords and pirates, he never wanted to take over the abbey or plunder some legendary treasure.
And this is where Slagar stands apart, what makes him the most compelling villain to me by a longshot. Because he's the only Redwall baddie motivated purely by emotion. He believes he was wronged, he's pissed, and he's making it personal. It gives his character a sense of weight and conviction even though his version of the truth isn't quite correct. It also makes him kind of suck at what he does. And that's actually fantastic.
Slagar has the distinction of being the only recurring Redwall villain over multiple books, which is kinda weird when you think about it, but I digress. In the original Redwall he was a minor character and the reader is led to assume he dies at the fangs of the adder Asmodeus. In Mattimeo it turns out he's survived, taken a new name, lost half his face, and changed careers from healer's apprentice to slavedriver. (Never too late to follow your dreams.) He's also determined to make all of this Matthias's problem, specifically, even though Slagar/Chickenhound and Matthias never even met during the events of Redwall. Does this make sense? Absolutely not. But it's perfectly logical if you're Slagar the Cruel.
What we do know is that there was a significant time skip between books, long enough for Matthias's son Mattimeo to be born and grow up into a hotheaded youth, and during that time Slagar was also staying busy. If we take the fox's account at face value (ha) then Slagar spent half a year just trying not to die while the adder's venom ate through his flesh. You can argue that he put himself into that situation in the first place, and he did, but it's still horrific. That's a lot of time to stew in pain and uncertainty and hatred. We know that Slagar wanted to kill Asmodeus for doing this to him- but by the time he was finally well enough to take action, the serpent was already dead by Matthias's hand.
And instead of going Hooray! Thank you so much for killing that horrible creature that was terrorizing the woods! Oh, you took care of Cluny too? The guy who killed my mom and left me for dead in a ditch when I was a kid? You're awesome, I owe you big time! Slagar, in his immature fury and unmatched pettiness, takes this as the ultimate insult. The idea of vengeance is what kept Slagar going this whole time and Matthias stole his kill! This will not stand!
It's funny when you deconstruct it like that but it's also the tragedy of Slagar's character. He could have continued pursuing medicine in his mother's footsteps. He could have been grateful to the mouse who saved Mossflower. But even though Slagar grew up and changed his name, he never really stopped being shitty little Chickenhound, who rejected the compassion the Redwallers showed him from the start. He was already full of poison before the snakebite and losing half his face just makes him double down.
Anyway, we aren't told exactly how he settles into his new role, but by the time we rejoin Slagar in Mattimeo it seems like he's already done this slave run at least once before and he's coming in with a plan. It's a solid plan, honestly, maybe the easiest time a baddie has ever had getting inside the abbey. Slagar can put on an act when it suits him. He's not stupid or incompetent by any means. But when he has the entire population of Redwall at his mercy, to conquer and dispatch or enslave at his whim... he snatches the kids and runs without even glancing at anything else.
Entire armies have crashed and broken upon these sandstone walls like waves time and again. Warlords have lusted after Redwall Abbey only to die without ever setting a paw inside. And this fox just walks in through the front gate, does a few magic tricks, roofies everybody, and leaves.
You can imagine Cluny and the other villains just screaming at him from the afterlife. You can also imagine Slagar kicking himself later on when Matthias gives chase and ultimately pursues him all the way to his death. He had the chance to remove Matthias as a threat, heck, he could have taken over the whole abbey when everyone was knocked out, but he never gave it a second thought. It wasn't part of the plan.
No, Slagar isn't stupid or incompetent- but he's short sighted to a fault because he's driven by emotion even when his scheme to gain entry was a clever one. He can't kill Matthias because he needs Matthias to suffer, and Matthias needs to suffer because he killed Asmodeus, and Asmodeus made Slagar suffer, and Slagar wanted to make Asmodeus suffer!! Duh!!!
Look, it would make perfect sense if you had six months to fume and seethe in agony while watching half your face slough off and wondering if each day would be your last. I find it really interesting that Slagar speaks highly of his mother's memory in Mattimeo when his parting impression of her in Redwall was basically "dumb bitch was too stupid to survive, not like ME, i'm awesome". We can only guess as to how much of Slagar's account is actually true, and how much is due to his warped perception or a flat-out lie... but it's clear that he has come to see himself as someone wounded and robbed, and everything that drives him now feeds back into his version of the narrative whether or not it's accurate.
If you're going to write an "insane" character you could do a lot worse than Slagar the Cruel. He doesn't hear voices or hallucinate or babble nonsense like some other villains in this category. He can make a plan and execute it- he can even muster a bit of charisma, though it's not his strong suit. Slagar's madness lies in his insistence that he is right, despite all reason and evidence to the contrary, and the obsession that makes him unable to deviate from his path. He can be logical, but his logic only serves the delusion. He can think quickly on his feet, but can't see far enough ahead to know when his scheme is doomed to backfire.
BJ gave us so much out of this side character, who only showed up in Redwall for a few chapters and was presumably killed. Imagine if every vermin rando who met an implied end had the potential to come back as a bigger threat later on! That would be so much more interesting than Generic Warlord #54752! As it is, Slagar stands alone- broken and messy and spiteful and compelling in a way that other baddies with no emotional ties to Redwall get to be. He's not actually justified in his revenge, but he thinks that he is, and that gives him more conviction than any general with an army at his back.
Other villains do more, but Slagar pulls it off with much less. The scope of his damage at Redwall is really quite small, but the deeply personal and violating nature of his plot is what makes him so nasty, why he feels like a major threat even though he doesn't want to conquer or kill his enemies. He wants to hurt them. He wants to screw with their heads. And there's no low he wouldn't stoop to in the interest of achieving his goal, because Slagar the Cruel is of course right!
TL;DR Shoutout to self-destructive mentally ill mangled men in capes, gotta be one of my favorite genders. This one just happens to be a talking fox. When I say I want more nuance in Redwall villains I'm not just talking about vermin who learn to change their ways, though of course we can never see enough of those. I mostly crave complicated villains who demonstrate positive qualities in the mix... but in the case of characters who are straight up evil, Slagar stands out as the only one who isn't motivated by greed or wanton bloodlust. And not only do you get to see exactly how he turned out that way, you get a glimpse of what he could have been if he made better choices, and you come to understand that his rabid tunnel vision is what leads to his downfall. Literally. He doesn't have a dramatic sword fight against Matthias. He fails to look where he's going and falls in a fucking hole. It sums up Slagar's character perfectly. You might even argue he's the worst Redwall villain based on that-
-but that is exactly why I think he's the best.