Villainous Widow's Peak

In real life, the widow's peak (a distinct point in the hairline in the center of the forehead) is a result of a lower-than-usual position of the intersection of the bilateral periorbital fields of hair-growth suppression on the forehead and is a dominant inherited trait. The term widow's peak is from the belief that hair growing to a point on the forehead is an omen of early widowhood.

For some reason, in fiction, it seems like it's also an omen of becoming the villain (or, at least, becoming one big Badass). Maybe because it makes one's hairline look like angry eyebrows.

Actually, in some villains this may be because of weird pattern balding, where the hairline recedes on either side of the center just a bit, as a sign of the stress of their job.

A form of Show the Forehead that can overlap with Forehead of Doom. See also Bald of Evil, where there's no hair left up top.

Examples of characters with a Villainous Widow's Peak include:

Anime and Manga

Vegeta from Dragon Ball, as seen in the trope picture.
Higuchi in Death Note has this kind of hairstyle. He's also a Corrupt Corporate Executive using the Death Note to kill business rivals.
Baron Ashura from Mazinger Z is The Dragon of the Big Bad and he/she has this hairstyle. It is not obvious from first because he/she always wears a cowl, but if you pay attention, you can notice indeed he has one.
Hamato Miwa (or Karai) from the 2012 version of the Teenage Mutant Ninja turtles has in human form a villainous widow's peak.

Comic Books

In Alan Moore's Top Ten story "Deadfellas", in which Hungarian vampires are analogous to Sicilian mobsters, the younger vampires laugh at the older "vidow's peak Vlads" for their horror-movie behavior and dress style, much as the Real Life "Mustache Petes" were derided and ousted by younger and less honor-bound mobsters.
The Joker.
The Riddler is more often than not depicted as having one.
Sinestro.
Raven actually grew one over the course of the New Teen Titans, eventually inspiring a plot concerning her father Trigon.
There are several characters from The Trigan Empire who have this, mostly villains.
Doctor Strange had one when first introduced (as a good-but-unnerving "master of black magic" who was meant to seem vaguely demonic). He still has it, but it's milder now that he's an unambiguous hero.
Ghost Rider foe Lilith is a pretty extreme case.

Film

Dracula is usually portrayed in films and other adaptations as having one. It's since a requisite part of the Classical Movie Vampire's appearance. Especially so since the Hammer Horror versions in which Dracula is played by Christopher Lee.
Grand Moff Tarkin. Well he is played by Peter Cushing....

Literature

Otto von Chriek from Discworld is not a villain, but he tries to evoke the Classical Movie Vampire look and therefore has one.
In Alexander Kazantsev's Destruction of Faena, the eponymous planet is inhabited by the two "races": the "longfaces" (humans who have the widow's peak) and "roundheads" (the rest), with the former generally being major assholes who believe themselves to be the master race. Kazantsev was probably satirizing the racism based on the skin color with this one.
This is a racial trait in the Dragaera series, in which it is called a "noble's point" and all "Dragaerans" (read elves) have them except for the Teckla, who are still elves, but are the peasant class. Not evil per se, but they have a tendency to be pretty ruthless, especially if you are a Teckla or human.
Mrs. Jeepers in the Bailey School Kids series is described as having one, and her students suspect that she is a vampire.
Oddly enough, Johnny Tremain's widow's peak is described in-story as a sign of wisdom. Presumably the trope has changed over time.
Oberyn Martel of A Song of Ice and Fire has one, and while not totally unsympathetic is a jerk who will go for Anything That Moves and is known for fighting with poisoned weapons.

Live-Action TV

Julian from Kindred: The Embraced.
Eddie Munster from The Munsters.
Dr. Chaotica from the Star Trek Voyager holodeck program "The Adventures of Captain Proton". As an Expy of Ming the Merciless, it's required.
The Master from Doctor Who had one of these in some of his incarnations. Most noticeably on Roger Delgado.
- They gave Roger Delgado that widow's peak for the role—I guess they thought it looked more sinister than a receding hairline. (They also widened the white streaks in his beard.)
In the Star Trek: Enterprise two-parter "In a Mirror Darkly", Mayweather's Mirror Universe counterpart has a widow's peak.
In Jekyll Hyde has a minor widow's peak, as opposed to Jackman who has straight hairline.
Evil Willow in Buffy the Vampire Slayer.

Video Games

Hikawa of Shin Megami Tensei: Nocturne has a very impressive one.
Kazuya Mishima of the Tekken series sports one, as well as the appropriately malevolent demeanor.
Maxie of Pokémon Ruby and Sapphire fame has this, though he's more a Well-Intentioned Extremist than anything else. There's also Lawrence III, from the second movie.

Web Comics

Sinfest depicts both the devil and his "fanboy" with one.

Western Animation

Although less pronounced than most of the examples on this page, all three members of the Fire Nation royal family have widow's peaks.
A young Dr. Wily had one in the Mega Man cartoon.
Though not as pronounced as other examples, Vlad Masters of Danny Phantom clearly has one, furthering his vampire-like appearance.

Other Media

Satan sometimes has one.
- Frank Zappa's 'Titties & Beer' features the devil as a little guy with a red suit and widow's peak.
- Marvel Universe villain and Satan-expy Mephesto has one.
Dr. Fu Manchu "with a face like Satan" is sometimes represented as having one.

Real Life

Gene Simmons of KISS has a very prominent one while in make-up.
Notably employed in the Real Life deliberately shaven sakayaki tonsure of historical Samurai, supposedly developed along with the topknot (chonmage) to allow a better fit when wearing a helmet. Younger Samurai were obliged only to shave and shape their immediate hairline into a highly defined widow's-peak, but older Samurai would extend the tonsure well past the crown, while retaining the widow's peak for as long as age-induced baldness would allow, sometimes growing their topknots into shapes that could be folded back over the tonsure, improbably making this style overlap with both Bald of Awesome and a comb-over. This fashion was gradually adopted by the wider Japanese culture in latter periods, but is now only seen, like topknots themselves, on sumo wrestlers.