Complete Monster/Mazinger Z

The Mazinger trilogy - Mazinger Z, Great Mazinger, and UFO Robo Grendizer - is rife with villains who are utter bastards. Given Go Nagai's penchant for Crapsack Worlds and Downer Endings, it should not be shocking. However, many of them fall into the Woobie, Destroyer of Worlds or Well-Intentioned Extremist categories or have a Freudian Excuse or some redeeming qualities (most of them avoided Bad Boss and You Have Failed Me... scenarios, and as bastard-like as King Vega was, he sincerely loved his daughter). And then there are these complete and utter lowlifes...

Mazinger Z

 * Dr. Hell certainly qualifies, especially if you know his Backstory: when he was young, Abusive Parents and a crappy childhood turned him into a Woobie, Destroyer of Worlds with a habit of Kick the Dog, a tendence to Murder the Hypotenuse and, a wish of purging the world of idiots and forcing the survivors to bow down to him. When he was older, he designed Doomsday Devices for Hitler and made human experiments with inmates in Auswitch. Several years after the end of WWII, he led an archaeological research expedition to the Greek Island of Bardos, where he found an army of Humongous Mecha, and right away, he murdered the whole expedition minus Dr. Kabuto. Later, he got himself an army of Mooks by mechanizing the brains of corpses and turning them into expendable Cyborgs programmed to serve him faithfully and unquestionably. He constantly designed weapons of mass destruction his Co-Dragons used to obliterate whole cities.
 * As far as Baron Ashura is concerned, "good" and "evil" are meaningless words made up by human beings. So he/she is perfectly capable of hitting children (once, he/she blew apart the wheelchair of a disabled girl to prevent her from escaping), murdering little dog cubs, or commit worse atrocities if he thinks it will further his/her goals (like machine gun several survivors of a shipwreck as he/she gloats they will be revived like cyborgs and turned into his/her slaves. Or like slaying a hostage or pawn after he or she has outlived his/her usefulness). He/she was easily Hell's most brutal henchmen and usually the most loyal, and he would do anything to turn Hell's dreams into realities. Under his/her command, his/her Mechanical Beasts set whole cities on fire, bombarded them with missiles until they were smoking ruins, caused earthquakes to bring them down, cut ships in two halves, murdering the whole passage, shot or blasted planes down, blew up trains and buses, and killed thounsands of people.
 * Before Hell met him and saved his life, Count Brocken was a Nazi officer who was considered particularly blood-thirsty by his own partners. He also was the first of Hell's henchmen to kill someone on panel on the manga even before Ashura, and he was capable of joking as he cut heads off - an Iron Mask is not quite to start with, but yet...
 * In one of the alternate realities from Shin Mazinger Zero, himself. He  tried to murder  Kouji, and  in front of an impotent Kouji.

Great Mazinger

 * Marquis Janus was a witch who was more than willing to befriend people under disguise before poisoning them. Another of her acts in the Gosaku Ota manga episodes was slicing Misato in half to pay Tetsuya back for defeating her in a fair fight.
 * Great Marshall of Darkness was downright cruel, specially in the Ota episodes: His first strategy was blowing up one city, and then warning he would do the same thing in Tokyo unless the Japanese Government destroyed the Fortress of Science and Great Mazinger. The Government caved in... and then Great Marshall razed Tokyo to rubble anyways. And another of his plans involved creating a Kill Sat to blow up any from the space any target he felt like annihilating. You can say
 * While he didn't qualify in the original version due to the fact that, as mentioned on the page summary, he genuinely loved his daughter, Gosaku Ota's take on King Vega in the manga adpatation has had that single redeeming quality of his stripped from him, leaving us with nothing but an utter bastard who is simply depraved, unrepentant, and pure evil at his core. As the ruler of the Vegan forces, King Vegan is a brutal conqueror who abandoned most of his own people to die on their doomed homeworld, simply because he deemed them unworthy of rescue. Conquering world after world, Vega had Planet Fleed annihilated, with the people either massacred, converted into soldiers to be used as Cannon Fodder, or having the women sent to "service" Vegan officers. On Earth, Vega commands widespread destruction from his forces, and when he takes command, he intends to set off every nuke in the world in order to exterminate all life on the planet, something which even horrifies his fellow Vegan commanders, including the especially ruthless Zuril.

UFO Robo Grendizer

 * Barados. Oh, my God. Barados. Like it can be seen in the Character sheet, he deliberately targeted women and children in order to exterminate the Fleed's population. And then is the "let's drop several hundred of children from three thousand meters from height" incident...