Though the revelation of Sabrina Online's pornography producer/star Zig Zag's Dark and Troubled Past (hint: it includes being sexually molested by her father) seems poised to give this for her overly flirty and sexual nature, it's subverted. Indeed, raising the idea that she has turned out the way she has because of her childhood abuse is Zig Zag's Berserk Button, and the very first time she reveals it results in a furious Don't You Dare Pity Me! rant from her, with her insisting that she's gotten over it.
Lord Tedd may or may not have one. Consider the facts that he is the most powerful known being in the comic and that the creator admitted that people wouldn't like his backstory because it is just that sad. The adults in the prime universe (which is a decade or so behind Lord Tedd's) seem to believe he has one though, and are going to a lot of effort to make sure their Tedd doesn't suffer the same fate. For example, Raven is reluctantly willing to take Grace on a hunt for a dangerous boar...until he finds out she's Tedd's girlfriend, at which point he freaks out, and only agrees to take her under extreme duress.
Raven: You will follow my orders without question. If there is danger, you will leave me to die. Do I make myself clear? Grace: But— Raven:Do I make myself clear?
Susan also had a couple of events that shaped how she was now. The event that shaped her Straw Feminist views on men was she catching her father with a naked women who was not her mother, and her mother being a Straw Feminist herself, this is also the reason why she dyes her hair dark-blue. She also used to be a literal Wide-Eyed Idealist before her trip to France where she and Nanase were hunted by a vampire-like Abberation, and Susan being forced to kill it. Thankfully after a couple of arcs, she has gotten some closure and a less strained attitude.
In Fans!, Alisin Oberf, during her dark days when she was dying of a rare disease, was, beneath her Perky Gothexterior, a self-loathing mess, who took pleasure in heavy bondage and sadism. Years later, after she was cured of the disease, but not her negative self-image, one of her "partners", Keith Feddyg, emerged as Alisin's greatest nemesis, using her own self-loathing to force her to become his sex slave and blaming her for his having become a psycho. Subsequent evidence indicates that he was alreadythere before Aly ever showed up.
Also, Alisin killed Robert's goldfish, which may or may not have something to do with the latter's eventual world dominating ambitions.
Joel from Concession has about three Freudian Excuses: his father left their company to his older brother, his sister was killed by his brother when they were young, and their parents blamed Joel for the murder and put him in an asylum where he was sexually abused by a doctor. There have also been a couple of throwaway gags which, if taken too seriously, could imply that his mother resents him for not being a girl. If there was a trope for Freudian Sue, Joel would probably be the posterboy.
Parodied and invoked In The Order of the Stick: Belkar at one point claims that his sociopathic demeanor comes from being mocked in his village, and that he became an adventurer to become strong enough to kill them all. However, he only says this to get free experience points for proper role-playing.
Also played straight with Redcloak, who lost his village to the crusading of the Sapphire Guard as a child. Subverted with extreme prejudice by Xykon, who has reveled in his wickedness practically from the cradle and eats villains that need a Freudian Excuse for their villainy for breakfast.
Might also count for Nale, Elan's evil twin and The Unfavorite to their father, General Tarquin. He's evil, but incompetent because of ego. What he wants nothing more is to be his own man, and step out of his father's shadow and manipulation. Tarquin agrees to this, since it was the only thing stopping him from avenging Malack.
Pops up twice in Kevin & Kell: Ralph constantly tried to hunt and eat his brother-in-law Kevin because he didn't want his sister to go through the same heartbreak he did when his herbivore lover died in childbirth. More recently, Desdemona Fuscus tried to push Lindesfarne and Fenton to marry earlier and earlier. This was because she was secretly a vampire bat, and didn't want Lindesfarne to find out and call off the wedding.
Flik's whole motivation. Since we see the problem before we see its results, it's rather differently played than normal.
Gary's virginity and sexual dysfunction are allegedly a result of his severely repressive ultra-religious parents and cockteasing girls. (And he lives with a nigh-professional cock tease and a very talented amateur cock tease, and lusts after a girl with penis-phobia. Um...) However, this has never been mentioned again since the early strips in which Gary told the story, and the writers may not consider it very important; Gary's problems don't need that much explanation — he's just a hopeless geek, with his geekiness turned up to eleven.
Off-White: She's not a villain, but the tragic death of most of her litter is at least partly to blame for Jera's attitude.
Vriska from Homestuck is partially a HUGE BITCH BLUH BLUH because her guardian was a giant spider to whom she had to regularly feed other young trolls or be eaten herself.
Bittersweet Candy Bowl is fond of this trope, partly out of the author's desire to make characters with realistic motivations.
Fridge Logic suggests that the reason Tess refuses to get close to Paulo is due to her similar past experiences with older students. She was hurt and doesn't want Paulo to be. The memories of what she went through will keep her avoiding the idea of trying it because she can't help but think she'll just hurt him if she does. Her moments of trying to be close to Paulo may be less likely in the future than we know.
Lucy was ostracized by boys and girls at a young age. She became less cheerful and got Yashy to love, as a coping mechanism. She's not so much unwilling to open up to others as unable to at this point. There's a lot of social trauma to work through before she begins to smile even half of the time. Hence, why her pets see her differently than everyone — she opens up to them.
Paulo's fear of barking and growling came from a somewhat-traumatic childhood experience based on them. His jerk-ish habits came from a jerk teaching him how to be "cool" as a child.
Abbey acting violent or particularly angry makes him break down at realizing he's acting like his father. Justified, in that his father was a wife-beater. Also why he took Abbey, rather than Abe, as his nickname. His father went by Abe.
The chapter "Pillow Talk" delves into why Jessica is so dead-set on making Tess miserable: Tess's then-boyfriend broke the arm of Rachel's little brother. When Jess tried to find a way to get back at Roger, Tess twisted her abortive first attempt to paint Jess as a slut.
Jamie from Khaos Komix is a foul-mouthed homophobe who bullies Alex for supposedly being gay and disowns his best friends after they come out to him as a gay couple. Then you learn that he was sexually abused as a child and has a mother who doused him in scalding water when she caught him Playing Doctor with another boy which has led to him having massive issues about sex and sexuality and you wonder why he isn't even more messed up. However, he actually criticizes the use of this trope by stating that he doesn't believe that his traumatic past excuses his actions and that he still has to work hard to redeem himself.
In Freefall, Edge spent his formative years in a warehouse surrounded by uninteligent machinery. Robots, like humans, need proper socialization, and he's turned out with an It's All About Me attitude and the complete conviction that he's always the smartest (and most important) person about.
Sticky Dilly Buns has Ruby, who was traumatised at the age of about 14 when she discovered that her sister Amber was a porn actress - by being exposed to one of Amber's videos at a teenage party. Okay, that involved her sister rather than her parents - but the problem seems to have been compounded by having to keep Amber's secret from their parents for the next several years. Fortunately, her "villainy" is limited to a fear of men and a very petulant attitude, but it makes her hard to live with.