“We Need to Talk About Kevin” is a horror movie for parents. Strip away the showy flashbacks — cutting between several pasts and several points in the present — and it’s “We Need to talk About Damien.” Because the only simple, the only satisfying way of explaining the monster at the heart of this nightmare is “He’s the spawn of the Devil.”

Except he isn’t. Co-writer/director Lynne Ramsay (“Morvern Callar” was hers), working from a Lionel Shriver novel, takes us into the confused, overwhelmed mind of a mother (Tilda Swinton) whose son has done something horrific. And as she staggers through every awful day after that, everything in her life reminds her of the many signs that something was wrong with that boy pretty much from birth. And how neither she nor those who didn’t see what she saw in him could figure out a way to prevent a disaster.

Eva is ostracized in her town, slapped by strangers in the street, forever scrubbing or grinding off red paint spattered over her vandalized house and car. Her memories let her escape — she’s back in her young, free-wheeling, traveling days, embracing the hedonism of the La Tomatina tomato festival in Spain, the romance that led to her marriage to Franklin (John C. Reilly).

But quickly, another memory arrives to drown the first one out. There’s Kevin, the baby who cried so incessantly that she would pause, next to a jackhammer, just for the relief of not hearing his screams. In Daddy’s arms, he was angelic. But mom saw the real Kevin — a smart child who refused to talk, a defiant kid who spent years refusing to be potty trained, a boy whose studied cruelty he perfected before his first day of school.

And Eva frets. Was it her fault? Did he sense her regret at the life she gave up? Did her distance from him create this monster?

Ramsay’s technique turns a straightforward Columbine Kid tale into a visual collage of effect without a singled-out cause, a jumble of regret and guilt, signs seen and not properly dealt with, a weak parent who empowers a mentally ill child. Even a slap from that angry, unnamed fellow parent doesn’t snap her out of it.

“It was MY fault,” she says to another stranger who tries to intervene.

But “We Need to Talk About Kevin” works on you. Maybe we think Eva’s right about her responsibility when she hisses at her kid, “Mommy was happy until the day little Kevin came along — you know that?” . Then we see the boy’s cunning, his manipulations. Mommy’s right to be miserable.

We watch the escalating outrages, and we wait for an animal to be abused. Because we remember that warning sign from psychology class.

The always-daring Swinton is great at sending the mixed signals Eva must deliver. She suggests passion, frustration and wariness. Her scenes in the present give Eve the timidity of a whipped dog, and she wins our understanding, if not our sympathy.

Two boys portray Kevin, and both Jasper Newell (younger Kevin) and Ezra Miller (teen Kevin) play the kid as if they’re auditioning for “The Omen.” Ramsay’s camera doesn’t suggest that this is just how Eva sees him — unconscionably cruel. This is the movie’s subjective reality — Kevin is a bad seed.

But “We Need To Talk About Kevin” still makes fascinating fodder for discussion for people who have outgrown horror movies. If you’re old enough to be a parent, you’re old enough to be chilled to the marrow by this depiction of dysfunction, worrying all the way home the signs you might have missed in your own child’s development.

MPAA Rating:R for disturbing violence and behavior, some sexuality and language

Cast: Tilda Swinton, Ezra Miller, John C. Reilly, Jasper Newell

Credits:Directed by Lynne Ramsay, written by Ramsay and Rory Kinnear, based on the Lionel Shriver novel.  An Oscilloscope release. Running time: 1:52