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.
Depending on how you view him, Tony Almeida.
Tate Langdon from American Horror Story: Murder House. 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.
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.
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.
Walter White 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.
Warren was on the receiving end of a lot of bullying growing up, mentioning how he used to cry himself to sleep.
Cold Case tends to feature lots of Tragic Monsters and sympathetic murderers, but a particularly gruesome example is the Serial killer in it takes a village. Surely someone who tortures, mutilates and murders young boys just because they are able to beat him at a video game couldn't possibly have any sympathetic qualities right? As it turns out the game is symbolic of a room in a particularly awful foster home where he spent his childhood. During which he and five other boys were made to stand naked on a square for days at a time by his counselor. He attempts a Crowning Moment of Awesome by standing up to said counselor. Instead of joining in the other boys gang up on himand cut his finger off. He targeted boys who reminded him of the ones who attacked him and at times seems to legitimately believe its them, even though its been over twenty years. By the time the detectives catch up to him, he's an incoherent mess who slips in and out of third person. He finally comes to his senses long enough to realize that his latest intended prey is just an innocent little boy who has nothing to do with what happened to him so he lets him go. As awful as his crimes were, even the detectives couldn't help but feel a tinge of sympathy for him, or at least for the little boy he was.
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.
Dexter has the Trinity Killer. Notable in that this trope is used to make the character more disturbing, rather than more human (the usual utilization of this trope).
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.
The War Doctor seems to be this, being responsible for destroying Gallifrey. Or is he?
The Twelfth Doctor temporarily becomes this in the final three episodes of Series 9 when No Good Deed Goes Unpunished results in a Trauma Conga Line. In "Face The Raven", Clara stops him from going on a rampage when she — his Morality Chain! — must die when her efforts to stop a plot go horribly awry, but it's clear that he can only keep his anger and anguish in check for so long. Next in "Heaven Sent", he is trapped in a torture chamber and spends billions of yearsand lives escaping. The culprits were his own people. He emerges at the Despair Event Horizon and hot for Revenge and succor in "Hell Bent". It turns out he underwent most of the torture to get the means to save Clara from her fixed-point-in-time death, and in pursuit of this Tragic Dream risks the entire universe's safety, heedless of the wishes and entreaties of others — all because he can't take it anymore! When he tries to mind wipe her to keep her safe, she tinkers with the device. Upon being told this, and uncertain whether the tinkering had any effect, Clara and Doctor agree to push the button together and let fate decide; ultimately, he loses most of his memories of her, but he accepts this fate as a proper punishment, having already realized/admitted the wrongness of his actions at last, and allows things — and himself — to return to their proper state. This is in many respects an uber-example of the trope because far from being a potential "Destroyer of Worlds", for a brief period the Doctor is actually headed to become Woobie, Destroyer of Time.
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.
Adam Monroe on Heroes his path of destruction fueled by his heartbreak over Yaeko.
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.
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 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.
Morgana from Merlin. She has magic, in a place were that is a death sentence. Aims to stop this but no one will help her. 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.
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 Parrilla is an absolute master of making kicked-puppy-dog faces.
Mr. Gold, a.k.a. Rumpelstiltskin, started out as a normal person. Then he crushed his lower leg so that he would be send home from the Ogre Wars, after being given a (self-fulfilling) prophecy regarding his then unborn son growing up fatherless, just as he did himself. His wife, along with the rest of the village, declare him a coward. She spends years abusing him, finally leaving him and their son, Baelfire, for Captain Hook. When Bae is 'drafted' (the Duke's men come into the village and steal children away to place on the battlefield) as canon fodder in the the same war at the age of 14, Rumpelstiltskin looks for a way to save him and is tricked into becoming The Dark One. The curse overcomes him and he first takes revenge on his abusers, then saves the surviving children from the battlefield. His son finds a way to go to a land without magic, believing this will save his father from the Dark One curse. Rumpelstiltskin, having experience with portals ( his father abandoned him via a portal when he was a small boy) of the type that Bae creates, is too frightened to go through. He regrets it immediately, and spends the next 300-plus years searching for a way to reunite with his son. He eventually helps and falls for a young Cora, and she rips out her own heart to avoid feeling anything for him in return so that she can marry a prince to make herself royalty, and to obtain power and revenge on those she believes wronged her. Years later, he falls in love with Belle, who attempts True Love's Kiss on him with no warning after being tricked into it by Regina, Cora's daughter. He kicks Belle out of the castle, and Regina kidnaps her, keeping her imprisoned while telling Rumpel that his True Love was Driven to Suicide. While in the Land Without Magic, finally having found a way to locate his son, Captain Hook shoots Belle in the back, driving her over the cursed town line and into Rumpel's arms, making her lose her memory of him. Later, Rumpelstiltskin sacrifices his life to keep his father from murdering Belle and Baelfire, and turning the town into Hell on Earth. He is later brought back to life, only to end up in the clutches of Zelena, the Wicked Witch of the West, in an attempt to save his son's life. He's kept as a prisoner and slave for a year, tortured physically and psychologically. His son ultimately dies to save the town, and Rumpel is forced to watch, then forced to watch Zelena brag about Baelfire's death, and kept from even attending the funeral. Later, oozing PTSD, he's overtaken by the Dark One curse and does some very bad things to try and remove himself from the influence of the dagger that controls his soul while under the curse. He's caught at it and is banished from the town by Belle, during the six weeks of which he finds that his heart is being consumed by the Dark One curse and unless he finds a cure, the human part of him will die and the curse will be free to use his body to do what is wishes without his consciousness and lingering goodness to hold it back.
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.
As a boy, Greg Mendell and his father ended up camping in the woods when the Dark Curse created Storybrooke. This let them enter the town where the mayor (Regina) decided to make him her adoptive son, whether he likes it or not. Greg escaped from Storybrooke but his father sacrificed himself to do it. Greg spent the rest of his life trying to find Storybrooke, and his father, again and every action he takes is to this end.
Ingrid, the Snow Queen spent most of her life fearing her powers over ice. One day she accidentally killed one of her sisters with her powers and her other sister imprisoned her in a magic urn, thinking she was a monster. This twisted her thinking so that she believes only those who have magic could love her and everyone else deserved to die.
Maleficent spends most of Season 4B trying to get revenge on Snow White and Prince Charming. Why? Their actions caused Maleficent to lose her baby daughter without ever seeing her face or knowing what happened to her.
Poor, poor Drizella. Her own mother, Rapunzel Tremaine, was willing to kill her and use her heart to revive her sister Anastasia, the latter of whom she does care about. She also has no friends in Seattle, with the exception of Henry and formely Gothel. Ironically, Drizella ended up being a Creator of Worlds, seeing as how she created Hyperion Heights through the new curse. Come "Secret Garden", and her mother sacrifices herself to save her.
Tremaine is also this to an extent, seeing as how her actions were caused by her husband Marcus and Drizella essentially replacing her with the latter' stepmother Cecelia when she was locked in a tower, along with Anastasia's death.
Helena from Orphan Black. 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.
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.
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.
In the Star Trek: The Next Generation episode "The Survivors", the Dowd Kevin Uxbridge abandoned his life-long pacifist beliefs and wiped out the entire Husnock race (of which there were about fifty billion in the galaxy) with a single thought after the Husnock attacked his home colony and killed his beloved wife. Immediately afterward he realized the extent of his crime and sentenced himself to the Cruel Mercy of living alone on the dead colony world with only a hollow copy of his wife for company and permanent reminder of his deed.
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.
Lucifer sees himself as this and it isn't without some justification. From the beginning, he was destined to become the Devil, something he, his brothers, and his father are implied in "Changing Channels" to have known ever since God made the universe, before Lucifer would have even done anything actually evil; it would also have to mean that he and everyone just mentioned would have had to know not only that Lucifer would become Satan but that his ultimate destiny would be millenia trapped in Hell's deepest prison, alone, only to be released to be killed by his adored older brother, Michael, to bring "Paradise." Seasons 10 and 11 go on to reveal that he was also given the Mark to bear by God after they sealed an Ultimate Evil known as the Darkness into it, which is said to have corrupted Lucifer and is known to cause a constant, overpowering urge to kill. Once God's favorite angel, Lucifer feels that he has been cast aside for a species that does not deserve such love and that he and his brothers are being toyed with by God for the sake of some messed-up cosmic play God has planned out. Lucifer has tried to regain his prestige in God's eyes, but has been irrevocably cast as a monster, with God refusing to take responsibility for his own part in Lucifer's fall and leaving him to his fate. As such, a monster is exactly what he has become. Despite it all, he still loves his family and tries to talk Gabriel out of fighting with him, only to be told off for "playing the victim" (even when it becomes increasingly clear that he does have a very strong case for having the odds stacked against him) and being forced to kill Gabriel in self-defense, obviously broken-hearted about it, and his one attempt to get himself and Michael off the chessboard was likewise rejected by Michael, who told him that he is a monster who destroyed their family and that he (Michael) needs to kill him, prompting Lucifer to eventually respond, "If that's the way it's got to be... then I'd like to see you try."
Castiel in the sixth season finale. Abandoned by God, at war with his brothers, rejected by both allies and his True Companions for doing what he has to do, and hopped up on the souls of millions of monsters and Eldritch Abominations from Purgatory, he declared A God Am I and demanded that everyone bow down and worship him, or else he would destroy them. It did not end up well.
As of the season 9 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 Tyke Bomb 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.
Demons in general qualify as this once you find out their backstory: they're destructive, sadistic monsters purely because they are humans who have undergone such horrific torture in Hell for so long that they literally lose their memories of everything except Hell and subsequently lose their humanity, turning just as depraved and warped as their torturers. How did they get to Hell in the first place? Maybe they made the unfortunate decision to sell their soul to a stranger at the crossroads for a wish. Or maybe the angels running Heaven decide they needed to be punished for something, barred them from Heaven, and sent them to Hell after they die. (By the way, crimes that warrant this range from getting tricked into doing something wrong to refusing to follow their screwed-up orders.) Or even just because the King of Hell has reapers on the payroll and can get ANYONE he wants condemned to Hell. Jeez.
Veronica Mars: Cassidy Casablancas was molested, belitted, picked on by his family, and abandoned by his mother. It causes him to go crazy and a blow up a Bus Full of Innocents.
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.