If you think that bunnies can’t be monsters, you haven’t been paying close enough attention. Below are some film and TV sources of a few of the scariest hares we’ve heard of.

Buffy the Vampire Slayer: If a centuries-old vengeance demon like Anya (Emma Caulfield) is deathly afraid of rabbits, perhaps there’s something to learn there. Anya wears a bunny costume for Halloween in Season 4 because she’s told that the idea is to dress as “something scary.” Buffy lore has retconned Anya’s leporiphobia into having some relation to the fact that she seems to have bred and raised rabbits back in the Middle Ages when she was human (see S07E05, “Selfless”), but we don’t think she needs a reason. As Anya sings in “once More With Feeling”:

Bunnies aren’t just cute like everybody supposes
They got them hoppy legs and twitchy little noses
And what’s with all the carrots?
What do they need such good eyesight for anyway?
Bunnies, bunnies, it must be bunnies!!
—Anya Jenkins, “Bunnies”

Donnie Darko: Like the film Donnie Darko itself, Frank is difficult to explain. In life, or in the timeline of the primary universe, Frank Anderson was a young man—the boyfriend of Donnie’s elder sister. In the tangent universe, however, Frank at first seems to be a schizophrenic hallucination of Donnie’s who tells him to do strange and destructive things. But Frank’s mission is more complicated than that—he is trying to help Donnie save the universe, basically, by fixing the timeline, though this will result in Donnie’s death. Although Frank has mostly good intentions, he’s still pretty much a scary ghost—specifically, one of the Manipulated Dead—who can move back and forth in time (see: Sparrow, Roberta; The Philosophy of Time Travel), and his rabbit costume is truly terrifying.

Monty Python and the Holy Grail: The canonical killer bunny in filmdom is surely the sentry monster of the lair of the Legendary Black Beast of Aaaaarrrrrrggghhh. Despite Tim the Enchanter’s warning that the innocuous-looking guardian of the Cave of Caerbannog has a “vicious streak a mile wide,” King Arthur and his knights approach it anyway and suffer the gruesome consequences. Forced to retreat, the surviving members of Arthur’s party manage to finally dispatch the fearsome creature using the Holy Hand Grenade of Antioch. (The Killer Rabbit of Caerbannog also makes a notable appearance in the Monty Python stage musical Spamalot.)

Supernatural: The sad tale of Chester the (probably not actually a) Child Molester should have ended when some goons, in trying to frighten him for suspected inappropriateness toward children, accidentally killed him. But Chester’s tale continued beyond his death. A misfit and kids’ party performer in life, the Ghost of Chester Johnson seeks vengeance by manipulating whoever has the misfortune of wearing one of his costumes. Bunny outfits do end up being creepy looking remarkably often, and the big, grimy rabbit head once belonging to Chester, the first of his costumes we see, is no exception.

Watership Down: The animated feature of 1978, based on the British novel by Richard Adams, may be remembered by those of a certain age whose parents foolishly allowed them to watch the movie when it was broadcast on American TV in the 1980s and thereby learned what violent creatures rabbits could be. General Woundwort, the main antagonist of the story, treats his enemies with a barbarity worthy of Game of Thrones.

The folklore of Watership Down also has its own lagomorph grim reaper known as the Black Rabbit of Inlé, a phantasmic flying steward of the dead.

Tormented (rabitto horā 3D): Tormented (2011) is a live-action 3D horror film from director Takashi Shimizu of  Ju-on (Grudge) franchise fame featuring some serious bunny creepiness that jumps right out of the screen.

Bad bunnies star in many, many other films as well. Below are some video links to trailers for just a few.

Animated films: Bunnicula (full short film); Wallace & Gromit: Curse of the Were-Rabbit.

B-Movies (NSFW): Many of these movies are available online, but watch at your own risk. Viewer discretion is advised for these trailers due to the usual low-budget horror gratuitous sex and violence. With Night Of The Lepus (1972) being a notable exception, most horrible hares in this genre are crazed (human) murderers in bunny suits.
Bunnyman & Bunnyman 2
Bunny the Killer Thing
Easter Casket
Easter Bunny, Kill! Kill!
Easter Bunny Bloodbath
Kottentail

Mean and scary bunnies make cameo appearances in some higher budget films as well:

Bill And Ted’s Bogus Journey: Ted “Theodore” Logan (Keanu Reeves) has a brief encounter with the Evil Easter Bunny in Hell
Twilight Zone: The Movie: The Rabbit Trick from the segment “It’s a Good Life”
One Crazy Summer: the cute and fuzzy bunnies opening sequence

 Alice: How long is forever?
White Rabbit: Sometimes, just one second. 

― Lewis Carroll, Alice’s Adventures in Wonderland