Aku's evil takes center stage at the midpoint of Season 5 of 'Samurai Jack'

The shape-shifting master of darkness himself. 

Who is Aku? 

The intro to the original run of Samurai Jack, narrated by Aku himself (then Mako, now Greg Baldwin, a great replacement), tells us almost everything we need to know about him: He is "the shape-shifting master of darkness" who "unleashed an unspeakable evil" into the world, inviting a challenge in the form of the show's title character. Jack nearly succeeded, but, "before the final blow was struck," Aku "tore open a portal in time, and flung [Jack] into the future," where Aku's "rule is law." The conflict at the show's center is Jack's attempt to "return to the past, and undo the future that is Aku." 

There is more to Aku than just this, though. In a two-part episode of the fourth season of Samurai Jack, we learn that Aku is actually a remnant of the primordial darkness thought destroyed by creation, taken root in the soil of Earth and feeding off its life. He is, as Jack describes him in this appropriately dark episode of Samurai Jack, "evil personified. The darkest soul from the pit of hate." To Aku, "all that is good and innocent must be decimated. That is his will, his desire." Aku is also often portrayed as a comic figure, all the more terrifying for the sadistic pleasure he derives from his tormenting. We've seen both sides of Aku in the show, his buffoonery and his psychosis. But now, at the midpoint of Samurai Jack's final season, we see his evil demonstrated more plainly than ever before.

The most obvious form of Aku's evil is Aku himself. This episode opens with a ruthless demonstration of his evil, and the power by which he abets it. The Scotsman (John DiMaggio, aka Bender from Futurama, Jake from Adventure Time, Drago from Kim Possible, and dozens of other roles; he's a member of the voice acting mafia in good standing), a ribald, exuberant warrior who became one of Jack's few companions despite (or because of?) being as different from him as one could be, has assembled an army to attack Aku's dwelling. (And he has brought his daughters, all tough warriors like their father, along with him.) It was wonderful to see the Scotsman again, even if his assault on Aku is an abject failure; Aku is simply too powerful to suffer defeat by any means other than the sword Jack must reclaim. In fitting Scotsman fashion, however, some of his novissima verba* do get under Aku's skin. But until Jack reclaims his sword, no one will get any further. 

Not even sword-wielding, wheelchair-bound, elderly Scottish man with a machine gun leg.

Yet Aku's evil takes other forms as well. There is the despoliation of nature that his reign has wrought. In stark contrast to Jack, who strives as best he can to live in harmony with nature, Aku replaces as much of the world's natural beauty as he can with perverse corruptions or mechanized horrors of his own making. In this, he somewhat resembles Saruman of The Lord of the Rings, he with the "mind of metal and wheels" in the Ent Treebeard's description. Aku's is the sort of evil that will destroy a forest of beautiful trees, but leave one as "a reminder of his power and oppression." It is precisely this aspect of Aku, and the contrasting behavior of Jack, that was the seed of Ashi's turning away from her brainwashed upbringing. 

Saruman's got nothing on Aku. 

Far more profound, however, is the evil that Aku has unleashed among the sentient inhabitants of the world. To have a brutal industrialized hellscape is one thing, but to populate it with criminals and overlords who assist Aku in creating "a world where the innocent and the weak are always preyed upon," in Jack's description, is entirely different. The true evil of Aku is that he has dehumanized the world, reduced the inhabitants of Earth to a Darwinian struggle in which strength and cunning untethered from morality prosper. It is a world not merely of "catastrophe, devastation, and carnage," as Jack says, though it is certainly that. More devastating, though, is that it is a world where vices have become virtues, where children have become slaves to evil and manipulation (of particular import to Ashi, raised as a brainwashed child soldier), where restraint and mercy lead only to death and ruin, where the few pockets of innocence and joy are singled out for ruination, where underlings do not merely carry out evil orders but torture for pleasure (calling it "fun"). This is the unspeakable evil Aku has unleashed, and it is enough, in this episode, to turn Ashi definitively away from the life she was born into.

But not before coming into her own as a protector of the innocent. 

Samurai Jack, however, is full of surprises. And thus it is in this episode that Jack, who, "after all he's been through, still can't bring himself to hurt an innocent," in the words of one of his tormentors, finally breaks. (Falsely) believing his actions have caused the deaths of dozens of innocent children who had been brainwashed into ferality, he accepts the invitation of the ominous horseback figure that has haunted his visions in previous episodes. It is a continuing testament to the virtuosity of this final season of Samurai Jack that I do not know what this means; it could be a sort of symbol for a yielding to the suicidal impulse that has tempted Jack in previous episodes, or to his descent into a dark version of himself. Whatever it is, though, it doesn't seem good. It seems, in fact, like some form of giving up, the ground for which had been laid in Jack's nihilistic reply to Ashi after she has finally decided that Jack is right, that Aku is the evil one. When she asks what can be done against Aku's evil, Jack says: 

Nothing. I've fought Aku for ages. I've seen countless innocents die. I've lived this nightmare for what seems like an eternity. There is no way to defeat him. There is no hope. No way out.   

The Samurai has come a long way, and his journey has weighed on him. But whatever happens to Jack, we must hope, for the sake of the world of Samurai Jack, that either he, Ashi, or someone else continues to fight against the evil that is Aku.

*That's "newest words," in literal Latin; we would say "last words." Yes, the Scotsman dies, a fact spoiled for me this morning by YouTube. But we haven't seen the last of him; he lingers as a kind of Celtic Force ghost. I hope we see him again.