Complete Monster/Inuyasha


 * Naraku. He is a half demon who wishes to become a full demon by using the sacred Shikon Jewel. How does he want to go about doing this? By messing up the lives of every single one of the main cast's lives because he can. He was responsible for every major atrocity in the series: Miroku's Wind Tunnel curse, Sango's brother becoming Brainwashed and Crazy, reviving a group of serial killers to slaughter his enemies, and sending his minions to kill villagers to lure the protagonists to him. He's a mastermind at manipulating others to do his bidding and he's killed Kikyo numerous times because he wants to remove the only side of him that could possibly be redeemable. If that wasn't bad enough, he essentially was responsible for Kikyo and Inuyasha's falling out and killed a huge margin of Koga's wolf clan. He's also not above killing his minions if they choose to disobey him and killing innocents if they even so much as become inconvenient to him. He doesn't have any ties to any other person, human or demon. All you can do is try to kill him. But what he can do to you...And to think he was this bad BEFORE he was Naraku. Becoming Naraku just gave him the power to be as Jerkass as he ever wanted. He never even truly loved Kikyo: he just really wanted to have her as his own.
 * Most of Naraku's offspring (aside from Hakudoshi and Akago or Juromaru and Kageromaru) are anti-villainous woobies that die peacefully - or at least redeem themselves before they die. Not so for Goshinki, Naraku's psychic homicidal offspring that murdered an entire village of innocents just to attract Inuyasha's attention. And when Inuyasha steps in to defend two newly made orphans, Goshinki breaks the Tetsusaiga and smashes him out of the air. Nobody cries when Demon!Inuyasha slashes him into a spray of gore on the landscape.
 * Tokajin, the cannibalistic sennin. To Inuyasha's disbelief, he wasn't a Youkai, but a human who learnt spells by eating his own master's flesh. He spent the rest of his life capturing mountain wanderers and keeping them as cattle before eating them. He feels absolutely no remorse for this, as he thinks humans are pathetic and weak. And he eats them very casually.
 * Mukotsu of the Seven-Man Army. In the anime, he was softened considerably, but in the original manga, he was a depraved misogynist, explaining to Kagome and Shippo that he loves to kill women with his toxic gas (although he clearly doesn't mind too much who else dies, as evidenced when he annihilates an entire village to demonstrate his poison), and then attempts to rape Kagome.
 * The Spider Head Leader who pretends to be a kindly priest to Nazuna, under the pretense of protecting her from the Spider Heads under his control, and killed her parents all for the Sacred Jewel in Inuyasha's possession. Also he planned to eat her and subject her to a Fate Worse than Death as Spider Heads are created from devoured victims.
 * Gatenmaru despite part of his name meaning heavenly and wearing effeminate lipstick and a pink haori and his men are really nasty victims of their own hedonism. Gatenmaru, himself leads a band of blood thirsty bandits who pillaged and had them slaughter an entire village of a kind old man and his grandson who had Inuyasha and co had saved while masquerading as a human, and these men had done these evil deeds willingly even after discovering their boss was truly a demon, in fact became even more enthralled in their actions. Gatenmaru feeds off women quite literally, by eating them alive through draining their essence through his tongue while showing sadistic satisfaction in their deaths. He does all of this Forced To Watch while he had poisoned and captured Inuyasha who had to watch helplessly as Gatenmaru mocked his weakness, referring to him as a lowly half-breed. Safe to say, nobody was upset when he and his men are gruesomely ripped to shreds as Inuyasha's demon blood awakened out of the sheer anger towards them.