Bloodbath Villain Origin

When a villain's origin involves the character (previously outwardly normal) suddenly flipping out and killing a large number of people in a single incident. Bonus points if any of these circumstances apply:

The villain was just a kid at the time.
The people killed included the villain's family.
The villain had no real reason to do this.

See also Where I Was Born and Razed. If the villain's origin involves a literal bath in blood, see Blood Bath.

Examples of Bloodbath Villain Origin include:

Anime and Manga

Naruto has Zabuza (1,3), Itachi (1,2, apparently 3 at first), Hidan (3, probably 2), Kakuzu and now Sasuke (1,2).
Fullmetal Alchemist 's Anti-Villain turned Anti-Hero Scar begins his serial-killing career by killing any and all available Amestrians - after waking up in a hospital to find that his entire family has been killed by the State Alchemist Kimblee and that his brother's arm has been grafted onto his body in the place of his own severed arm. He flips out and starts his rampage with the two Amestrian doctors who saved his life. Who are married. And who also happen to have a daughter who'll grow up to become a pretty damn good automail mechanic. Oh, the Humanity!!
Sensui's Start of Darkness was like this: he flipped out upon learning Humans Are the Real Monsters, and killed everyone in the Black Black Club's villa.
- Coincidentally, the people he murdered were themselves enjoying a demon blood bath.
There's quite a few sympathetic starts of darkness in Rurouni Kenshin, but the one closest to this trope is when Seta Soujirou murders his abusive parents after they pushed him one time too many and ended up as The Dragon for a Social Darwinist.
Rob Lucci of One Piece is a perfect example. His first mission as a CP9 agent was given to him when he was only a kid; to save some hostages from pirates... he killed not only the pirates, but all the hostages as well, because they were getting in the way/didn't deserve to live. And he got a nasty-looking scar on his back in the process, which coincidentally looks like the Marine flag.
The Mad Pierrot from Cowboy Bebop.
While the extent of her evil is very debatable, Evangeline McDowell from Negima is convinced that she underwent this.
Sometimes the bloodbath happens because the person in question got pushed too far and snapped out. Just take a look at Lucy from Elfen Lied and what she did to the cruel kids in her backstory for beating the puppy that she'd started caring for to death.
- Bonus point #1, minus points for having two separate flip-out incidents before it became a habit.
Alas, poor Yomi... (2, and when she's lost it, 3)

Comic Books

Evil-Ernie develops a lust for killing after being exposed to the Dream Probe, and murders an entire street of people.
- and after being treated with Neurotech, which helped unlock his supernatural powers, he goes on another rampage!

Film

In the remake of Halloween, young Michael overhears that he's going to get some massive psychological intervention soon to try and curb his nascent Serial Killer tendencies. He decides to take advantage of his freedom at the moment and it being Halloween to exact revenge on: The bully who teased him about his mom being a stripper, his abusive dad, overbearing sister and her boyfriend.
- In the original, Michael just snapped and murdered his sister for no apparent reason. Doctor Loomis eventually concludes there's nothing to him, except evil.
Although we don't hear all of Gallaxhar's back story, he does directly state that he was the one who blew up his planet.
Depending on how much damage you think he did, Tai Lung.
In Star Wars:
- Anakin Skywalker's Start of Darkness (murdering an entire camp of Sand People) and later his attack on the Jedi Temple. The slaughter of the Tusken Raiders was because they tourtured and killed his mother.
- Palpatine/Darth Sidious, who throttled his family and their bodyguards to death, and intended to at the very least kill his father since he was born due to hatred of him, and for no reason other than his family forbidding his contact with Plagueis, as revealed in Darth Plagueis.

Literature

Subverted in Harry Potter, with Sirius Black.
- And, at the same time, played straight with Pettigrew.

Live-Action TV

The Criminal Minds episode "Haunted" begins with a man having a psychotic break at a pharmacy and killing a number of the other customers.
Rumpelstiltskin in Once Upon a Time goes on a killing spree after suddenly acquiring awesome powers. Justified by the fact that they were all soldiers trying to conscript his barely 14 year old son, and he was trying to protect the kid - but he was still evil, since the power was dark and corrupted him.

Tabletop Games

Forgotten Realms: Sammaster's fall started in somehow similar way - he blasted slavers with spells, but in process also caused death of the very people he tried to resque. Not that he was the shining example of mental stability before this.
In Warhammer Fantasy Battle, one of the named chaos villains used to be a shaman's apprentice, the weaker of two twins, constantly bullied and ridiculed for his weakness. When his prayers attracted attention of Chaos god Tzeentch, he was granted powerful magic as well as a powerful body to match (he was magically grafted on top of his brother's shoulders) His first act was to turn his power against the tribe, making the village "run with molten flesh".
Table-Top RPG Eclipse Phase includes "The Lost Generation", an attempt at accelerated maturation Gone Horribly Wrong that are all assumed to be this. Several of the survivors of the program ARE this, but not nearly all of them.
Very common angst-oriented origin story for both good and evil Werecreatures in Werewolf: The Apocalypse.

Video Games

Joe Darke in Ace Attorney, a business man who after accidentaly running over a man with his car, entered in panic and started to kill the witnesses of the accident (and the witnesses of those murders too).
In F.E.A.R., Paxton Fettel fell into this trope as a kid during the so called "synchronicity event", during which he was possessed by Alma Wade and killed several scientists using his psychic powers.

Web Comics

Helen Narbon in Narbonic... insofar as she can be considered a villain. Apparently all mad scientists try to cause as much carnage as possible when they go mad and get full control over their mad science abilities. Mind you, some of them were already evil before they were mad.