These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Johann Schmidt from the main continuity is a Bastard Understudy to Adolf Hitler himself. Once a bitter, psychotic street kid, this eventual Diabolical Mastermind began as a petty criminal with a violent streak. After having his romantic advances rejected by a Jewish girl who'd been nice to him otherwise, Schmidt flew into a rage, killed her, and finding he liked murder, set out to commit it again. Convincing a friend to try and assassinate Hitler, Schmidt stepped in and saved Der Fuhrer's life. He then donned a Skull-shaped mask and took on the role of a grateful Hitler's spymaster. Trapped in suspended animation by Cap, Schmidt awakened in the modern era and resumed his old ways. Firmly convinced that Dystopia Justifies the Means and that everyone needs somebody to bully, Schmidt has committed every crime in the proverbial book, from the petty to the grandiose. He made several attempts at gaining World Domination through the use of the Cosmic Cube (killing thousands of people every time), attempted to transplant Adolf Hitler's brain into Captain America's body, tried to bodyjack Cap himself on several occasions, and fought The Kingpin for control of the New York drug trade. He has manipulated the Scarlet Witch as part of a plan to exterminate the world's mutant population, used his Dust of Death to gruesomely kill any subordinate who fails him or looks at him funny, and has been the mover and shaker behind countless Neo-Nazi movements, fascist governments, and terrorist cells, most notably HYDRA and AIM. In a Bad Future he became God Emperor and spent his time triggering natural disasters, then showing up to inform people that he would not be saving them. When his daughter, Sin, was born, Schmidt planned to kill her for the crime of being a girl. While in a relationship with Mother Night, he brutalised her constantly. With an end goal of reducing society to its most primitive, dog-eat-dog levels, Schmidt is universally despised in both the superhero and supervillain communities.
The Ultimate Red Skull is even WORSE. Ultimate Skull is the son of Captain America, and therefore has a version of his powers. He's a killing machine, although that doesn't sum up the sick delight he takes in what he does. His list of atrocities includes: killing two hundred people, followed by skinning his own face at the age of seventeen, training terrorists and working with numerous historical monsters, including infamous mass-murderer and cannibal Idi Amin, shooting Kennedy, just to send a message, and using the Cosmic Cube to force the members of A.I.M. to eat each other. And then there's his most infamous moment: he gives a woman (later the Red Wasp) a Sadistic Choice: kill her husband with a pair of scissors or he'd kill her baby. She did it. The Red Skull then chucked the babyout a window and let his three thugs gang-rape her.
In The Avengers: Earth's Mightiest Heroes, Skull, the leader of HYDRA in World War 2, kidnapped mystical creatures to use them to kill the Allies. When the Nazis lost, the Skull opted to kidnap Captain America's teen sidekick Bucky Barnes and torture and brainwash him into the amnesiac assassin the Winter Soldier. In modern times, the Skull continues his dreams of conquest and hatred of Captain America by unleashing a chemical compound to kill the heroes and civilians. The chemical has been engineered to cause slow and painful death by making the victim's face turn into a grotesque parody of the Skull's own.
Hilarious in Hindsight: The original inspiration for the Skull? Joe Simon saw a hot fudge sundae shaped vaguely like a human torso. He'd originally planned a villain called Hot Fudge until he realized the cherry looked a lot like an exposed skull. So...
Moral Event Horizon: As a young man when he murdered a teenage Jewish girl for rejecting his clumsy and creepy advances, even though otherwise she liked him and was willing to overlook it. He thought it was the best rush of his life and it served as a release for all the pent up anger and frustration he felt towards the world. One of the reasons he joined the Nazis was for more opportunities to have a rush like that, and it still serves as one of his prime motivations for his numerous crimes.
Unpopular Popular Character: In universe he's quite possibly the most hated character in the Marvel Universe, outside of it, he's widely considered to be among Marvel's greatest villains.
Famously, in a Marvel/DC crossover, the JOKER refused to have anything to do with him.