The film is based on a novel by Lionel Shriver. I haven't read it, but from the extensive Wikipedia text on it, I gather that it sticks to the same myth. An evil child and two naive parents.
Kevin is portrayed as a little demon already in infancy, out to get his mother. How could he, and why would he?
At the end of the film, the mother asks Kevin why he did his monstrous deed, killing his father, his young sister, and a number of high school students. He has no answer. That's not surprising, since the parents should be asking themselves.
When a child becomes a demon, the explanation can almost always be found in its childhood, especially the first few years. That's the time when the parents rule the child's cosmos. So, if we want to know why a child commits devilish deeds, we need to scrutinize the parents. They're probably not going to confess any guilt in the matter, so there's no point in just asking them about it.
The film would have been so much more interesting and believable, if it examined more closely how the parents really treated their son. There are tiny clues, now and then, about the mother being cold and distanced to the boy – but it's only presented as if the boy is to blame, because of his cruelty towards her, already when he's an infant.
But she must have been the one initially triggering her son.
When the story refuses to go there, it commits the same treason to children that the adult world keeps repeating and repeating, anytime the new generations aren't fulfilling the dreams of the older ones in every detail. Utter nonsense. The adult world has total power over the children, so it's preposterous that it admits no blame for how they turn out.
Also, it's bad drama, simply because Kevin's behavior remains unexplained, as if coming from a whim of the gods or some kind of mental malfunction nobody could do anything about.
Well, the old deus ex machina, the surprise divine intervention of which all humans were innocent, has a new form. Now it's explained as a medical condition or a genetic malfunction. But experience tells us that even such things need triggers to burst out into action.
I would love to see another version of We Need to Talk About Kevin, where we see it from his viewpoint, through the years of growing up. How did his mother treat him, really? That story remains to be told. All through the 112 minutes of the movie I waited in vain for it. That was the real horror of it.
PS: I still have to say that I marveled at the performances of the main actors. Even the two children playing younger versions of Kevin, Rock Duer and Jasper Newell, were doing so well with their evil characters, I wouldn't dare to ask how. But the main fire of the film was kept alive by Tilda Swinton and Ezra Miller, playing the mother and son with sparkling brilliance, making the unspoken words echo throughout.
Also the director, Lynne Ramsay, impressed me. She served the dishes slowly, careful with the spices, sort of like Hitchcock, knowing how to play with the imagination of the audience instead of splashing special effects. Furthermore, I have a feeling that she chose to film it all in a very subjective perspective of the mother, because she was not convinced the story would be the same from other angles.