There’s no shortage of mangled, mauled, or mutilated women in horror, the genre that transforms bedrooms and bathrooms, cabins and churches, schools, pools, and anywhere else a girl might go into the last place she’s seen alive. But in at least one regard, horror is women’s best friend: it’s the one genre that delves regularly and meaningfully into the thorniest questions about motherhood — a proud tradition that extends back to “Rosemary’s Baby” and includes more recent efforts like “Mama,” “Proxy,” and “We Need to Talk About Kevin.” By erasing the line between the monstrous and the maternal, writer-director Jennifer Kent makes an important contribution to that subgenre with “The Babadook,” her feature debut. An exceptionally rich psychological portrait of a woman (played by the superb Essie Davis) horrified by her impulses to abuse and even murder her extraordinarily difficult six-year-old son (Noah Wiseman), it’s an enormously compassionate and sadly relevant film about misplaced blame and the often impossible demands of motherhood. Even before the mysterious arrival of a scarlet storybook at their home, Amelia (Davis) has trouble keeping her monster-obsessed son Samuel (Wiseman) from acting out at school and at home. Widowed and shut out of the circle of local moms after Sam accidentally breaks a classmate’s nose, Amelia, a onetime writer, is exhausted by having to care for the elderly from 9 to 5 and for her son when she gets home. Sam’s hyper-realistic ways of acting out — taking homemade weapons (to fight monsters) to school or pushing a girl out of a tree house when she taunts him for not having a dad — make him a truly difficult kid to take responsibility for, let alone to have around. Restless and creative and innocently unaware of his ability to injure others, he’s such a challenge that Amelia begs a pediatrician for a prescription that will knock him out so she can finally have a week’s worth of uninterrupted sleep. She’s on the brink of a breakdown when Sam’s newest nighttime read, a handsome but unsettling pop-up book called “The Babadook,” informs her that, once she realizes “what’s underneath, [she’ll] wish [she] were dead.” Amelia immediately rips up the tome and its Edward Gorey-esque illustrations of a pale ogre in a Victorian top hat, a heavy coat, and Freddy Kruger-esque claws. But it returns to her doorstep with added pages, this time intimating that she’ll kill the family dog, then her son. The Babadook soon reveals itself to Amelia aurally: whispers in her ear, the buzzing of a swarm of flies, the yawning crackle of something very large opening its jaw. She isn’t sure whether she’s just imagining the black shadow darting across the ceiling or not, but Sam is convinced that it’s real, and that it’s going to hurt them. But Amelia soon discovers that it wants the same thing she does: for Sam to die. As menacing as the unseen monster is, “The Babadook” bases its scares on the increasingly aggressive relationship between mother and son. Amelia feels just as true-to-life as Sam does; she could have been an outstanding mother if her son were normal, if she could afford more help in taking care of him, if her husband hadn’t died and left her a single parent. She knows all this too, which makes her dangerous. When she finds shards of glass in her soup but not in Sam’s, she’s all-too-willing to believe the worst about her son. Because both characters feel so genuine, we’re as much afraid for Amelia as we are of her after she’s possessed by the Babadook and stalks after her son with a knife. Sam eventually begins to retaliate with his monster-fighting weapons, and the possibilities of how things might end for them are as heartbreaking as they are chilling. It’s to Kent’s enormous credit that, even when Amelia does the most unthinkable things to her child, she never feels like a monster, but instead a desperate woman who’s so blanched of energy and creativity and hope that she just can’t think of anything else to do. “The Babadook” is the rare horror tale that’s also a triumph of empathy.