Thank goodness Justin Long has stuck around. After breaking out in the frightfully gross road movie “Jeepers Creepers” and popping in aughts comedies like “Dodgeball” and “Accepted,” the 45-year-old has been on screen long enough to be getting genre-movie roles tailor-made for him — or at least uniquely suited to his strength — playing (not so) “nice guys” in all their dishonesty.

Having tested the waters as an East Coast professor on probation in “After Class” and an idiot Hollywood hotshot facing a #MeToo storm in last year’s “Barbarian,” Long follows up as the big bad in the horror comedy “It’s a Wonderful Knife,” released in theaters Nov. 10, before streaming on Shudder down the road. As the murderous mogul Henry Waters, who slices down anyone that stands in the way of his town’s redevelopment, Long steals the show once again. His have-at-it fun and eerie eyes provide a manic glee that the rest of director Tyler MacIntyre’s oddly tempered slasher riff on Frank Capra’s Christmas classic could stand to match.

MacIntyre is clearly ecstatic to have Long onboard. Waters is the first character to appear, whispering like Dorothy Michaels through a wide toothy grin and sporting a seasonally inappropriate tan. “We got Justin Long!” the film seems to triumphantly announce. That makes it all the more intriguing when Waters is unmasked by the plucky high schooler Winnie Carruthers (Jane Widdop) before the opening title card even appears. After a bloody murder sends a high school party into a panic, Winnie ends up saving her brother Jimmy (Aiden Howard) by killing Waters.

A year later, the freak encounter has ended up revitalizing Winnie’s town. The streets are in high holiday spirits. Winnie’s brother and father (Joel McHale) become an altruistic real estate duo on the rise. It’s merry and bright for everyone — except Winnie, who is frustrated by her family’s ability to move on from the terrorizing incident. And her boyfriend is cheating on her!

Whether that appears to be enough for the teenager to wish for a world where she’d never been born seems to be a stretch that “It’s a Wonderful Knife” has little investment in justifying. Every movie has a title, and it must be lived up to. So, like Jimmy Stewart’s George Bailey, Winnie erases her existence and discovers that she’s pretty important to those around her. Her neighborhood becomes a hedonistic dump overflowing with foreclosed businesses, with the money-grubbing slasher she had killed now serving as the local mayor.

Working under some modest resources, the team shrewdly focuses on realizing the small-town setting. Production designer Tiana P. Gordon finds some humor redressing sets for this more miserable reality. The Carruthers family’s already ugly suburban home becomes a messy, cavernous den of infidelity, ruled by a decrepit, wine-drunk Mom (Erin Boyes).

“It’s a Wonderful Knife” is most inspired by such now-this-is-that revelations, but writer Michael Kennedy’s screenplay frequently gets bogged down by an impulse to overexplain a plot that should be familiar to anyone who understands what the title is referencing. Winnie and her budding crush, the high school pariah Bernie (Jess McLeod), have conversations that run in circles, flattening what should be a winning queer romance. The blunt-force dialogue can lead to some amusing moments though: “I had one kid! Now I have none!” McHale bellows in a failed attempt to break from his otherwise well-deployed smarm. The line is the 28th or 29th clarification of the premise.

MacIntyre’s direction is unfussy enough for “It’s a Wonderful Knife” to recover from such awkward moments. But the frequent stabs at horror are just as easily forgotten. The genre slant promised by the title seems to be less of a tonal responsibility than an excuse to abruptly break out into the occasional suspense set piece.

The film is defined by an overruling superfluousness — it’s the sort of thing people might be satisfied with watching from the corner of their eyes while wrapping presents. At least with the inclusion of Long, who returns to take center stage for the climax (naturally), it finds the momentum to get to the finish line. Even after the viewer puts all the gifts under the tree and discovers there’s still 20 minutes left, Henry Waters is a strong enough reason to see things through.