Disgaea has had at least one complete and utter monster throughout its history; we will have to turn our attention to Vulcanus from the first game. This loud and brash angel uses humans and angels alike to attack and terrorize the Netherworld and attempt to hurt innocent demons. Why? He believes that all demons are completely evil, that's why! And that's not all, he also believes that Humans Are Bastards and angels are superior in every way. When Laharl and co. pilot Space Battleship Gargantua off to Celestia, he sends them to Lunar Snowfield instead, just to keep them from interrupting his plans. And in case that's not enough, he even makes a pact with some of the most dangerous and ferocious demons in the Netherworld - which, of course, is a violation of the rules of Celestia. Thankfully for everyone, in the end, he learns that he was an Unwitting Pawn in the Magnificent Bastard Seraph Lamington's plans and, after a failed attempt to kill Lamington (which shows that this guy is completely beyond redemption), gets turned into a flower (or a frog in the anime) by him as punishment for all the sins he committed. It's very interesting to note that this guy is one of the few Complete Monster villains to be Obviously Evil, as evidenced by his face and the fact that Laharl says he'd make a good Vasal if he wasn't an angel.
And Disgaea Afternoon Of Darkness' Etna mode adds even more to his complete monsterhood; he poses as God in an attempt to manipulate Flonne into killing Etna. And if that's not enough, he kills Maderas after he is defeated and attacks Etna. Very satisfying that he's sent away somewhere as punishment for his actions after his defeat.
In the credits of the Good ending, he faces another punishment of taking lessons from Flonne, implying he was changed back. Maybe this means his complete monsterhood in the game might be subverted. Nevertheless, he was quite the monster in the anime as he wasn't changed back in the ending there. That's saying something.
Forced Love reindoctrination...by the very angel he tried to kill multiple times...yeah, karma ain't letting you off the hook that easy, bitch. And to rub more salt in the wound, she now has his old job and another angel decided to take his name. Fucker deserved it.
On that note, the only final villain in the Disgaea series who doesn't come close to complete monsterdom is Nemo, who actually has sympathetic reasons for being a rat bastard. So where does that leave the other two?
The false Overlord Zenon somehow avoided death at the hands of the real deal during an infamous battle, most likely due to being Not Worth Killing. Rather than rethink his priorities, he decides to take Zenon's name and hunt down its reincarnation. The search takes him to Veldime - namely, Snow Melody - where he abducts a young girl (Rozalin) and incinerates the rest of the village! He may have left survivors behind (Fubuki, who went blind; Yukimaru; and their elder, who died prior to the story's beginning), but he wound up drawing the attention of Serion and Shura (Adell's parents) - when he defeated them, he used his powers to bind them to his will, denying them their memories so they cannot act to bring about his downfall (and this, as you might expect, ruined Adell's childhood forever). But wait! It gets even worse! Once that is settled, he uses his powers to rob humans of their memories and corrupt their forms, turning them into demons as a result (Adell himself is a demon and is thus immune); even the offspring of altered humans (like Taro and Hanako) are corrupted in turn! At the end, he states that he only came to Veldime for the reincarnation of Zenon, to pamper and restrain her so she may never regain her former power - once he achieves that end, he's more than happy to wipe out human consciousness in Veldime before fleeing the resulting Netherworld. His comeuppance, you ask? Rozalin, having becomed disillusioned to his motives, willingly unseals Zenon...who then obliterates him.
Super Hero Aurum is just as racist as Vulcanus against demons, killing them and their Overlords with aplomb - he may have had good motives to start out, but that all went to hell after so many years. Eventually, he found that he was running out of strong Overlords to kill and wanted to either face one more or just face death. It's the methods that cement him as having descended into complete monsterdom. He asked Mao, who just had an argument over his Slaystation Portable being ruined by his clumsy father, about any weaknesses the Overlord may have. Mao tells him only on the condition that daddy just gets beaten up - Aurum breaks this promise and kills him stone dead. Then, he disguises himself as Geoffrey to raise Mao according to his own twisted ideal after the boy lashed out in an appropriate manner, effectively turning him into a basket case - and when Almaz comes along, to ensure the boy doesn't relapse, he sets an Obvious Trap for his royal girlfriend so that he'd take it in her place and die - and for the cherry on top, said boy was his number one human fan. Let me repeat myself - all for the sake of defeating one more Overlord or dying gloriously in battle, he engages in near Mind Rape of one kid and cruelly kills off another. In the bad ending, Aurum discovers that Evil Is Not a Toy as Mao throws a Suicidal Cosmic Temper Tantrum. In the good ending, Mao denies the death wish and takes Aurum as his latest guinea pig. However, he fucked up, he deserved what he got - you know you're a sick bastard when the demons are disgusted with your work.
