For Baron Ashura, "good" and "evil" are meaningless words made up by human beings. He/she is perfectly capable of hitting children (once also having blown apart the wheelchair of a disabled girl to prevent her from escaping), murdering little dog cubs or commiting 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 or her usefulness. He/she is easily Hell's most brutal henchmen, usually the most loyal, and would do anything to turn Hell's dreams into reality. Under his/her command, his/her Mechanical Beasts set whole cities on fire, bombard them with missiles into smoking ruins, cause earthquakes to bring them down, cut ships in halves, murdering the whole passage, shoot or blast whole planes down, blow up trains and buses and thus kill 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 is the first of Hell's henchmen to kill someone on panel on the manga even before Ashura, and he is capable of joking as he cuts heads off.
In one of the alternate realities from Shin Mazinger Zero, Dr. Kabuto himself. He murdered his son Kenzo, tried to murder his grandson Kouji, and raped and murdered Sayaka in front of an impotent Kouji.
Marquis Janus is a witch more than willing to befriend people under disguise before poisoning them. Another of her acts in the Gosaku Ota manga episodes is slicing Misato in half to pay Tetsuya back for defeating her in a fair fight.
Great Marshall of Darkness is downright cruel, especially in the Ota episodes: His first strategy was blowing up one cit 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 caves in - and Great Marshall razes Tokyo to rubble anyways. Another of his plans involves creating a Kill Sat to blow up from the space any target he feels like annihilating. You can say Dr. Hell became an even greater bastard after dying and being brought back to life like a servant of Mykene Emperor.
King Vega's idea of what to do when your homeworld is becoming unstable and on the verge of destruction is to begin conquering and colonizing another worlds. Original inhabitants of a planet are usually exterminated, although in some instances they are "only" enslaved. To put his mindset in perspective, when his own daughter Rubina asked him why he isn't willing to make peace with the people of Earth, he couldn't come up with an answer.
Barados, as it can be seen in the Character sheet, deliberately targets women and children in order to exterminate the Fleed's population, which is without mentioning the "let's drop several hundred of children from three thousand meters from height" -incident.