You’d better watch out. You’d better not cry.

And if you’re thinking of a holiday mass murder and kidnapping, you’d better not try. Especially if it’s Christmas Eve.

Because Santa Claus is a jolly elf with — as Liam Neeson and his ilk always boast — “particular skills.” And he will open his magic toy sack of whupass on you if you keep him from his appointed rounds.

“Violent Night” is a Santa’s Slaughterhouse comic thriller, a sadistic, sometimes funny and seriously mean Santa movie for a seriously mean age.

The laughs and horror movie-level killings compete sentimental touches in a lurching, whiplash-inducing holiday “treat” from the Norwegian filmmaker who gave us “Dead Snow” (skiers vs. Nazi zombies) and “Hansel & Gretel: Witch Hunters.”

David Harbour stars in this attempt at merry mayhem, a gruesome action comedy that tries to graft machine gunnings, impalings and wood-chippings with “Santa is real, you guys!” sentiment.

It’s kind of bizarre. And it kind of half works. Until you think about it. Until you figure out you’ve had enough.

We meet the not-so-jolly man in a British pub, knocking a few back on Christmas Eve, a sad, weary and cynical gift-giver whose tab is picked up by a fellow “Santa” who “just got off.”

You’re not driving, are ye mate?

“Well, I steer a little.”

This Santa, like so many cinematic Santas, is figuring this is “my last year.” Only his reasons are greedy, spoiled kids, cash-crazed like their parents, and 440 million too many store bought cookies and way too many glasses of soured milk.

But one stop awaits that will give Santa flashbacks to a time before he brought presents to all the good girls and boys via “Christmas magic.” It’s not the “greedy, rich a–holes” of the Lightstone Estate in America who will change his life forever, possibly even end it. It’s the greedy, murderous a-holes who storm the place and threaten one very good believer and “Home Alone” fan, a little girl named Trudy (Leah Brady).

Tommy Wirkola’s movie “lurches” because there are a stunning number of starts and stops. We pause meet the estranged couple (Alex Hassell and Alexis Louder), Trudy’s parents, who re-unite to take her to this gated mansion full of vile one percenters. We meet the shrill sister of the husband (Edi Paterson), her vapid actor-hunk beau (Cam Gigandet, damned funny), her obnoxious rich “influencer” teen (Alexander Elliot).

The film spends time establishing and over-establishing the back-stabbing family dynamic, with most of them taking their queues from ruthless and foul-mouthed patriarch (Beverly D’Angelo, also funny).

But they’ve barely put the “I still believe” Trudy to bed when Santa shows up, complaining about reindeer and when and where they poop, and starts working his magic. He’s barely raided the bar when all hell breaks loose and a team of eight killer commandos slaughter the staff — the armed and the unarmed — and take the place over.

We’ve already lost track of the body count when John Leguizamo makes his entrance as their leader, the “Mr. Scrooge” who code-named his murderous minions Sugar Plum, Krampus and the like.

“Bah humbug, Mother F—–s!”

As Santa tries to “not get involved” and just be on his way, he must face the pangs of conscience that call for him to help the only real “innocent” in all this — Trudy — and his own past and present.

And the viewer has to decide how far down this rabbit hole of grisly “comic” violence we want to go with the fat man and his increasingly blood-spattered white beard.

Leguizamo makes a decent villain, properly pissed-off at having his cunning plan interrupted, absolutely furious that some of those who survive Santa’s first wave of kidnapper purges start to believe in “Christmas magic.”

Harbour summons up a droll, modern-world weariness and grim and violent sense of purpose in this drunk whose origin story involved spilling a lot of blood the ancient Scandinavian way.

The jolly man can take a beating, and a stabbing and strangling or three, and keeps coming back for more.

The “Home Alone” references set up some amusing booby-trapping with terminal blood-letting as its pay-off.

But all this lurching from Trudy’s true belief and her willingness to risk “the naughty list” by lapsing into profanity and helping help Santa creatively slaughter the slaughterers gets to be a bit much. There’s something discomforting about this particular sentiment mixed with senseless violence.

That slows the picture to an action-beat followed by a dead-pause rhythm that kills the pacing and kind of spoils “the fun.”

Because when you pause to pick up what some of the less interesting characters (and blander actors) are doing, one can’t help think of this week’s, or last week’s or the week before’s mass shooting and wonder if it’s just “greed” that has Santa burnt out.

One can join in the chuckleheaded chuckling over “Violent Night” only as long as you don’t think of how repellent it is, and wonder what the hell it is you’re laughing about.

It’s not really a holiday action movie “escape” when you’re not really escaping the gruesome gore and inhumanity the movie is all-too-giddy about showcasing.

Rating: R (Some Sexual References|Language Throughout|Strong Bloody Violence)

Cast: David Harbour, Beverly D’Angelo, Cam Gigandet, Alex Hassell, Edi Patterson, Alex Hassell, Alexis Louder, Leah Brady, Mitra Suri, Stephanie Sy, Alexander Elliot and John Leguizamo.

Credits: Directed by Tommy Wirkola, scripted by Pat Casey and Worm Miller. A Universal release.

Running time: 1:41