Smallville have a very large number of villains, and while many earlier ones are Ax-Crazy Psychopaths, middle and later seasons have introduced a lot of Tragic Monsters and these.
Hidden: Gabriel Duncan's father spent a long time trying to convince people of the mutating effect of kryptonite but failed and ended up being a meteor freak himself, and asked his son to kill him. Feeling that the plague of the meteor infection will grow out of hand after the second, larger meteor shower, he decides to nuke the entire town. Not before warning, and when she attempts to talk him out of it, kidnapping Chloe, though.
In season eight, they managed to make Doomsday to be something like this because of the horrible life Davis, his human host, has had. He was abandoned as a kid as opposed to having lovingfoster parents like Clark, and is cursed to bring nothing but death and destruction.
Adam Monroe on Heroes his path of destruction fueled by his heartbreak over Yaeko.
Dr. K in Power Rangers RPM. She unleashed a sentient computer virus that almost certainly nuked the planet, and is confirmed to have wiped out all of civilization outside of one city. Her motive? Escape from the top-secret government think-tank she had been trapped in all her life. In her defense, the guards caught her before she could set up a firewall. So she unintentionally killed most of humanity in an attempt to escape unjust imprisonment. And since the firewall was intended to keep the virus from escaping from the computer network in the first place, some consider it the indirect fault of the guards who kept her from installing it, thus guaranteeing spread of the virus.
When Tenaya 7 invades her lab, she mocks her, saying, "You pride yourself on how smart you are, don't you? But you still royally messed up, didn't you?"—after which the usually cool Dr. K completely loses it and angrily starts firing her sound cannon in random directions. After the flashback ends, she appears emotionally exhausted, most likely because of the guilt she faced for her mistakes, thus cementing her status on this page.
The poor, confused robot deliberately driven insane by a fringe political group in "Robot" is a clear example. It's forced to kill people, murders its own father, has a Freak Out, decides humanity is fundamentally corrupt, and then attempts to cause a nuclear holocaust to kill everyone on Earth apart from Sarah Jane as she was the only person who ever treated it with basic respect.
In the episode "Dalek", the eponymous creature suffers several existential crises in a row, from being (it believes) the Last Of Its Kind to being part human thanks to the method of its healing. Ultimately, it is Driven to Suicide over the pain of emotions it never even knew existed.
An in-universe example from the episode "The Doctor Dances":
The Doctor: All that weapons tech in the hands of a hysterical four-year-old looking for his mummy.... There isn't a little boy born who wouldn't tear the world apart to save his mummy - and this little boy can.
The new series characterises the Master this way - outrageously evil and destructive, but due to the Time Lords intentionally tampering with his brain when he was a child, leaving him permanently tormented by hallucinations of war drums.
Warren was on the receiving end of a lot of bullying growing up, mentioning how he used to cry himself to sleep.
In Legend of the Seeker, Nicci is even more of this trope than in the books, as her Knight Templar tendencies are downplayed and more emphasis is put on her desire to destroy the world.
Played with in Stargate SG-1 episode Absolute Power with Daniel Jackson, the show's typical Woobie of various other types. In the episode, Daniel is trying to sensitively obtain information about the Goa'uld from a small child, who touches his head in an apparently hostile act, as it renders him unconscious. When he awakens, he proceeds to have all of the needed knowledge revealed to him and proceeds to build the weapon that would apparently defend the Earth, but at the cost of having become at least a Jerkass Woobie if not outright evil. He fantasizes about very violent things until, finally, his goal is met. He then proceeds to take over the most powerful weapon in the world from his Supervillain Lair and destroy Moscow before waking up and realizing that having absolute power to defeat the Goa'uld in one fell swoop is not such a good idea after all. He wakes up to discover that it was All Just a Dream, but during the episode, it was implied that Woobie, Destroyer of Worlds is actually simply a part of his subconscious, with or without special knowledge. Not surprising, really.
Warehouse 13 had a textbook example in H.G. Wells. It started out with her daughter being murdered, who proceeded to hunt the killers down and cause them more pain than she felt. Then, after being turned into a statue for a century (while still awake the whole time), she found today's world even more lacking, and decided to explode the Yellowstone supervolcano to start an ice age. In a twist similar to the Buffy example above, Ho Yay saved the day.
The Greeed from Kamen Rider OOO are monsters whose MO is consuming the world and are generally causing massive pain to everyone. Then we find out why they're doing it. Their 10th Core Medal was destroyed, leaving them sentient, but even if they get all 9 of their remaining Cores, they have a void inside them that can never be filled due to the destruction of the 10th. They've been that way since they've become sentient, they're only trying to consume the world to fill a void inside them that they were born with. If that hunger is enough to drive them to the extremes they go to, then they definately qualify.
Lucifer in Supernatural sees himself as this and it isn't without some justification. He was a loyal servant to God, who felt that he has been cast aside for a species that did not deserve such love. He has tried to regain his prestige in God's eyes, but has been irrevocably cast as a monster. As such, that is what he has become.
As of the season 6 finale, Castiel is a straighter example. Abandoned by God, at war with his brothers, betrayed by his allies, and, worst of all, rejected by his Nakama, Dean and Sam, and hopped up on the souls of millions of monsters from Purgatory, he has declared A God Am I and demanded that they bow down and worship him. This cannot end well.
As of the season 10 finale, Dean. His entire life has been devoted to saving others, to his own detriment more often than not, with no credit, or appreciation from anyone but his fellow hunters (even his own family constantly abandons him and yet expects him to clean up their messes), he finds out he was genetically engineered by Heaven itself, was raised as a warrior  with little love shown him by his borderline insane father and now, after literally allowing himself to be murdered to keep from becoming a monster he's the First Knight of Hell, the Demon that all other demons fear, more powerful than the King of Hell and only less powerful than Lucifer himself and all because he wanted to save the world and keep the angels and demons from having their gang war on his planet.
Walter White in Breaking Bad could be interpreted as this. He is an extremely intelligent and skilled man, but works as a high school chemistry teacher and has to watch his former classmates and business partners from Caltech become multimillionaires off the company he co-founded. After a lifetime of this kind of indignity, without ever fighting back, he gets diagnosed with lung cancer; this humiliation collectively pushes him over the edge into cooking meth so he can make back the money he lost and get even with a world that continually screwed him over.
Jesse Pinkman is one, himself. Oh, you can taunt him all day, everyday: for years. And, he'll just take it. With, perhaps, the odd, nasty prank in retaliation, but nothing more. The minute you press his Beserk Buttons of 1) harming people he cares about, 2) harming children and, in the worst case scenario, 3) harming or threatening to harm children he cares about, however... you will find out that all that taunting? Has created an unstable, dirty nuclear device with a dodgy timer. Which you have just primed. He won't care about the consequences: he'll take you down as and when he is able to fix solidly on your position and got a clean line of sight. And, the fallout won't be pretty or discriminate, however low-yield he appears in comparison to others on this list.
Veronica Day, who manipulated her boyfriends into killing their parents because someone killed her birth mother before she had a chance to tell her who she was, and if she couldn't have a happy family, then no one could. And on top of that, she was caught because everyone thought she killed her mother. She was freed on a technicality but redeemed herself by confessing to one of her victims.
A mother who goes on a shooting rampage on her child's birthday/death day, who has a special grudge against "heroes" because not only was her child's injury and death ignored due to a police officer being killed in the same incident (they were caught up in a car chase), her own husband was a workaholic paramedic who didn't even take time off for his own son's birthday, eventually divorced her, and seemed too quick to get over his son's death (he hadn't, he just didn't show it as much as she did).
A father whose son was a Serial Killer, and blamed his "rotten genes" for what happened, kills any young man who happens to have the same glasses as his son, but shifts targets to the father of one of the victims who has been harassing him for years eventually, hoping for a Suicide by Cop. He also attempts to Mercy Kill his very ill wife, but is unsuccessful.
Another father who, after the death of his wife, planned to murder-suicide himself and his three sons by sneaking pipe bombs into their backpacks and sending them off to school, a field trip, and the hospital where their mom died.
A young woman who, as a teenager, was repeatedly raped and beaten by her father and brother, and when her doctors finally managed to convince her to go to the police with it, she found her mother already at the station denying everything. She snapped and began believing that all women should suffer what she did (and, presumably, what she wanted to do to her mother), recruiting her weak-willed, toadyish husband to do the actual raping part.
A woman who abducts other women, injects them with a paralyzing drug that will eventually kill them, and plays with them like dolls...because she walked in on her pedophilic, psychiatrist father giving away her favorite toys (given to her to keep her quiet while he was abusing her) to his latest patient/victim.
A comic book artist Forced to Watch as his pregnant fiancée was raped to death in front of him by a street gang. The trauma causes him to have episodes where he blacks out...during which time he, without knowing it, becomes a crazy skilled Vigilante Man, butchering members of the gang with katanas.
The final perp on Law & Order: Criminal Intent turns out to be a woman who runs an internet company (a sweeter, female Mark Zuckerberg Expy) who, tipped off to a break-in at her headquarters, returns to confront the offending party and finds that he has killed her boyfriend. He runs at her and she retaliates with a scissors to the neck. She actually breaks down into Tender Tears when confronted with the evidence, and it is clearly intended that our sympathy should be with her.
One suspect on Law & Order: SVU, who suffered from a terribly-painful terminal health condition, started a website on which she advised other people on how to commit suicide. Her willingness to facilitate suicide even in the clinically-depressed made her morally culpable in several preventable deaths.
In Fringe, Walternate. His son was kidnapped. His world is in the process of tearing itself to pieces, and millions of his fellow human beings have been killed. He's also waging a shadow war on our universe, since he thinks that it's the only way his can survive.
Interestingly, in the alternate reality where Peter died as a boy (an Observer did not save him), Walternate is better than Peter initially assumes him to be.
Tate Langdon. He's mentally damaged, neglected by his parents, and clearly depressed...and also a seemingly sweet boyfriend. And he cries so prettily! On the other hand, he's a mass-murdering, stalking ghost.
Richard Harrow is an expert killer, utterly broken by his experiences and injuries in the trenches of the First World War. His only friend, Jimmy, is a gangster who has a use for Harrow's skills. Harrow has therefore murdered a couple of dozen people on screen, many of them without even asking why.
Also, Jimmy is revealed to be one in season 2.
Holtz in Angel. He was a vampire hunter whose family was brutally murdered by Angelus and Darla to torture him. Except for his daughter, whom they turned into a vampire so Holtz would have to kill her himself. His guilt and rage caused him to abandon reason for vengeance and pursue Angel to the twenty-first century and manipulate people for his own ends.
Jack Bauer almost winds up becoming this. Throughout the entire series he's been betrayed, used, exploited, and almost all his friends and loved ones have wound up dead. In season 8 his last potential love interest is killed and the President betrays him which proves to be the last straw, sending on him on a Roaring Rampage of Revenge to expose the conspirators behind the day's events which in turn lead to his love interest dying and then mercilessly slaughter them, which turn out to be former President Charles Logan and Russian President Yuri Suvarov. What sets him apart from most of the others is he does still have some rationality and does overall still refuse to harm any innocent life; when pressured into the fact that going down this path would lead to several casualties in the ensuing war between the United States and Russia, he realized what he was doing was in the wrong. He may have stopped himself after realizing what he was doing would kill more innocent than guilty, but prior to that he pulled some pretty risky moves, including opening fire on a crowd of civilians in order to get to said corrupt President which could have backfired horribly and killed someone. I'd say he qualifies.
Regina, a.k.a. The Evil Queen is a powerful sorceress who has murdered several innocent people, has an irrational grudge against SnowWhite, and wipes the memories of everyone in her world because of that grudge. But she was also raised by a heartless queen who murdered the servant boy she was in love with after a young Snow was manipulated into revealing their relationship. Also, the adoptive son she genuinely loves is disgusted with her past and (at least at the beginning of the series) wants nothing to do with her. She tries to reform, but things always seem to get worse for her when she does. And it helps that actress Lana Parilla is an absolute master of making kicked-puppy-dog-faces.
Mr. Gold, a.k.a, Rumpelstiltskin,. He started out as a normal, but weak-willed, husband, but when he left the Ogre Wars because of a self-fulfilling prophecy in order to go home to his new son, his wife practically forsook him because of the shame of being married to a "coward". Years later she left him and their son, Baelfire, for Captain Hook. When Bae became of age to be drafted for the Wars, he became The Dark One to prevent this, which he did. But he became cruel with his power, enjoying taking revenge on those who walked all over him before, and when his son begged for them to go a land without magic, he couldn't bring himself to go at the last second, and only Bae went through the portal. Rumple immediately regretted this, and has been trying to find Bae ever since. He eventually helped and fell for a young Cora, but she took out her heart and married a prince instead because she wanted power and revenge. Later he fell in love with Belle, who Regina kidnapped, but made him believe was Driven to Suicide. And this was all before the series proper started.
Jefferson, aka the Mad Hatter. He's pushed into what he does by Regina's manipulation and betrayal, plus love of his daughter. Also, he's been driven more than a bit crazy by his time in Wonderland and being one of the few conscious people in Storybrooke.
Morgana from Merlin. she has magic, in a place were that is a death sentence. aims to stop this. is turned on by her former friends, as they ignore her and keep things she really should have been told. Merlin is the worst in this.
Helena from OrphanBlack. A clone, separated from her birth mother and from her twin sister Sarah, raised in a convent before being abducted and turned into a Tyke Bomb by a religious psycho which lead her to start performing large ritual scarifications on her back, was forced to murder her fellow clones one-by-one, and when she finally found the strength to rebel, she was taken by another bunch of religious nutjobs who forced her to get married and performed invasive procedures to steal her egg cells in a very rape-y scene, before one of them flat-out tried to murder her. Oh, and that twin sister she feels a connection to ? She impaled her on one occasion, and shoot her at point-blank range on another. The one and only character that ever showed any kindness to her was Kira, Sarah's daughter.