The entire past storyline of We Need to Talk about Kevin is told subjectively. There are any number of devices along the way which indicate this but the what makes it absolutely certain is the final scene in which Eva asks Kevin why he did it. It's at this moment that, if you hadn't realised it earlier, it becomes clear that her remembrances of the past have been an attempt to ascertain what it was that led him murder his fellow students.
Even without the final scene there are many signifiers to tell the audience what's going on. Flashbacks often begin or end on a close-up of Eva's face - letting us know that we're seeing the past through her eyes. The camera almost never leaves Eva's side and when it does the resulting images are so heightened, so much more theatrical than the rest of what we're watching, that they leave little doubt that what we're seeing are fevered imaginings.
That's not to say that her POV is only one we get although it is the one that overwhelms the film: As she sees it Kevin was, from day one, a sociopath, a monster, a demon seed hell-bent on destruction. He never wanted to play ball with her and he only pretended for the rest of the world. However enough gaps are left in the narrative to allow other perspectives to bleed in. We never actually see Kevin commit any violent acts leading up to the school killings. We don't see him destroying Eva’s room so we don't know if he really did it maliciously or if he genuinely wanted to make it prettier and we are being mislead by Eva's perception of the conversion beforehand. More pointedly we never see if Eva put the drain cleaner back in the cupboard and so don't know whether Kevin blinded his sister or whether it was a genuine accident.
As such Franklin's view, that Eva's antipathy for Kevin that makes him the way he is around her, is leant credibility it wouldn't otherwise have. Eva is shown to suffer from post-natal depression: perhaps it is her impatience and frustration with Kevin that drives him away. The audience sees times were she completely snaps - no matter how much sympathy they might have for the struggling mother dealing with the completely obnoxious child the way she throws him off the bed is genuinely horrifying.
Furthermore Kevin is allowed to suggest that his personality is the result of his mother's own abrasive behaviour and the film backs him up - it's no accident that Eva and Kevin are dressed nearly identically on the day of the massacre. It' also no coincidence that the school massacre occurs the day after Kevin catches his parents discussing a divorce that is, for all intents and purposes, the result of his presence. To be sure he is shown to be preparing earlier but perhaps his difficult home life is the tinder needed for him to explode.
Still, as I asserted above, it is Eva's point of view which dominates the film. This leads to a film which, from a dramatic perspective, is tiresomely repetitive: is there a way Kevin can defy his mother? He will take it. Is there a way Kevin can destroy something beautiful? He will destroy it. Is there a living thing with a connection to him? He will kill it. Ultimately, despite the uncertainty created by its subjective approach, We Need to Talk About Kevin is a film of absolute certainty. Yes, we may never know why Kevin committed a massacre but as the present day storyline and the last, repeated shot of Eva happy in the rain with Franklin suggest there is one thudding certainty: When Kevin came along he destroyed Eva's life.