One of the great contributions by Stan Lee, Jack Kirby, and Steve Ditko (among others) to superhero comics was the idea that the heroes were just like us. They looked heroic and had great power but inside they had feet of clay. The human melodrama found in these early Marvel stories didn't just extend to the heroes, but their villains as well.
In some stories, villains were allowed to have monologues where they aired their grievances with the world. They would expound on their histories, detailing tragic childhoods and the bad hands they had been dealt in life. Sometimes they even succeeded in winning the reader's guarded sympathy. Marvel's roster of tragic and strangely human villains has only grown since the '60s.
10 Mole Man
Yes, Mole Man (or Harvey Rupert Elder) is a megalomaniacal little creep. Yes, we've seen him repeatedly try to conquer the surface world with his cadre of subterranean monsters and an army of moloids. Yes, we've watched as he couldn't take a hint and try to annoy Squirrel Girl into dating him with a series of elaborate evil plots. That being said, it's hard not to feel sorry for someone with low enough self-esteem to choose to name himself Mole Man.
His origin story in Fantastic Four #1 makes this painfully clear. Elder, born with a hunched and mole-like appearance, became embittered by the rest of humanity for disregarding him for his looks. His hatred grew to the point he left society to venture underground and become a monarch beneath the Earth's surface.
9 Gibbon
Martin Blank, a mutant orphan born with ape-like agility and simian features, just wanted to be somebody. Spending his entire life mocked for his appearance, Blank wanted to be admired as a hero and after some encouraging words from our Friendly Neighborhood Spider-Man, he decided to give it a try.
In The Amazing Spider-Man #110, Blank, now calling himself "Gibbon," approaches Spider-Man with an offer to become his partner. Caught off guard, Blank's proposal leaves Spidey in stitches. This was the final insult that pushed Blank over the edge, and he proceeded to attack Spider-Man and start on a path to villainy. Luckily Blank's innate goodness has shone through over the years and he's been able to prove himself an unlikely hero several times.
8 Taskmaster
Taskmaster can be a real nasty piece of work. We've seen him work as a merc-for-hire for some of the worst of the worst Marvel has to offer. His workmanlike approach to villainy makes him a fun villain but not exactly worthy of sympathy. That being said, over the years some revelations about his past make him far more three-dimensional.
See, Taskmaster's photographic reflexes take a toll on his mind. Specifically his long-term memory. The space his brain uses to store the fighting styles of hundreds of heroes and villains takes away huge chunks of memories about his past life, including his wife and possibly his daughter. From his occasional flirtations with anti-heroics, maybe Taskmaster has just forgotten the good inside him?
7 Toad
Like several others on this list, Toad was born malformed and was subsequently abandoned by his parents. Struggling with a learning disability and constant mockery for his appearance, he developed unwavering loyalty to anyone who would show him even a little kindness. unfortunately, one of those people was Magneto, who was then undergoing his "melodramatic mutant terrorist" phase.
Sychophantic to a fault, early issues of The Uncanny X-Men saw Toad taking heaps of abuse from Magneto and the rest of the Brotherhood of Mutants (to be fair, he acts pretty gross towards Scarlet Witch...) Thankfully, over the years he has begun to find a much more emotionally healthy place among Marvel's mutant community.
6 The Shocker
The Shocker (Herman Schultz) just wants to get ahead in life. He doesn't want to rule the world, or get revenge, he just wants a quiet and comfortable life in the suburbs. Sure, he's willing to steal and blow stuff up with his vibro-gauntlets but compared to someone like the Red-Skull, that's nothing!
What makes you feel sorry for Shocker is that he's aware of his low place on the villain totem pole. He's a punching bag for heroes and other villains. His self-awareness and modest goals make him among the most human of Spidey's rogues' gallery.
5 Man-Killer
Katrina Van Horn, The Man-Killer, was handed a real raw deal in life. She was originally an Olympic skier who had to put up with a constant barrage of sexist put-downs from the male athletes in her field. During a race with particularly misogynistic skier Karl Lubbings, her opponent cut her off causing her to plummet off a cliff.
As seen in Marvel Team-Up #8, her severe injuries were treated by outfitting her with a powered exo-suit, and she recovered with enhanced strength and a militant hatred for men (Keep in mind, it was the '70s.) Writers could probably do a very poignant take on her origin story if she were to make another appearance.
4 Ghost
We've all had jobs we didn't love, but hopefully, none of us have had as tough a time at work as the man who would become Ghost. Making his first appearance in Iron Man #219, it took decades for Ghost's origin to be parceled out to readers. While we don't know his real name, he was allegedly a brilliant but shy programmer who spent years making his company millions while receiving no credit.
After deciding to take a long-deserved vacation against the wishes of his bosses, he ended up falling in love with a young woman who would later die in a tragic accident. Driven by grief, he developed a way to merge himself with the computer network he had been building, only to discover that the woman he had loved had been hired by his bosses to seduce him and keep him amenable to their demands. This launched him on a crusade to take down the corporate world, by any means.
3 Songbird
Running from an abusive home, Melissa Gold found a new family and name "Screaming Mimi" among the group of female wrestlers called the Grapplers. When they were denied the money and benefits afforded to their male counterparts, the Grapplers turned to crime and were subsequently defeated by The Thing and several other heroes in Marvel Two-In-One #56.
Joining up with Baron Zemo's Thunderbolts, then masquerading as heroes, Gold took the new identity of Songbird and began to become accustomed to the public's adulation. She began a tumultuous romance with her fellow Thunderbolt Mach-1 (a disguised Beetle,) and confided to him that her abrasive demeanor was the result of feeling abandoned by everyone and everything she cared about.
2 Googam
Googam, son of Goom, is one of those wonderfully ridiculous giant monsters from Marvel's years just before the Silver Age. His father Goom was a conqueror from the Planet X who failed to take control of Earth, and his son infant son Googam was sent to pick up where he had left off.
He was unsuccessful in his mission, and Googam would spend his life struggling with his father's disappointment in him. His humiliation grew deeper after he had to be rescued by Doctor Strange during the Monsters Unleashed crossover event. Luckily, a pep talk from the good doctor allowed Googam to make peace with his issues with his father.
1 Doctor Doom
Doom is above the pity and sympathy of lesser humans. This is true. However, looking at the past of one of Marvel's most diabolical minds paints him as a victim of his endless pursuit of power. Orphaned by the Baron of Latveria's soldiers, the brilliant Victor Von Doom found himself put on the path to super-villainy almost by forces beyond his control.
While we've seen him do no shortage of unspeakable things (Remember when he sent Franklin Richards to Hell in Fantastic Four vol. 3 #68?) there's a sense of wasted heroic potential around Herr Doom. He could save the world if he put his mind to it, but he's just too preoccupied with proving he's better than Mr. Fantastic to do it.