Harrowing stuff, but, as Douglas Adams once said, sit back, relax, and be harrowed.
The film starts as a stream of consciousness, almost. Tilda Swinton is going about her business, living a pretty nasty seeming life. She's in a run down house, which someone has attacked with red paint; goes for an interview at a run-down travel agent, is confronted by people in the street who hit her and call her names, and virtually every second of this seems to trigger a flashback of some kind to her past, when she seemed much more affluent, is married, and has a child; this is Kevin. So from the outset we know two things; something has happened in her life which has resulted in her being broke, alone, and hated by the community, and that she has a son with whom she fails to bond with from the start, even before he's born.
At the start of the film, this is a very jumbled mess, and I have to say, half an hour in I was thinking "Well, someone needs to talk about Kevin... and soon please?" but as time passes, the vignettes become longer, and more coherent, as I suppose your recent memories are, and we are gradually clued in to what actually happened.
In a way, this flashback device is initially frustrating, with snippets of knowledge arriving in seemingly random order. However, this isn't an accident, and is part of the power of cinema; yes, it's frustrating, confusing, and disquieting, but that's no accident, and being glued to your seat for two hours, it forces you to take your medicine, and eventually this builds into an understanding of the situation that goes beyond the narrative.
It's a film of two key performances, and three actors; Tilda Swinton is pale, awkward and emotionally devastated in a way that I suspect only she can be; by contrast, Ezra Miller as Kevin the teenager (and you know, I really wish that phrase didn't constantly pop into my head) and even more impressively I thought, Jasper Newell as Kevin as a young child, possess a dead eyed confidence, indifference and malevolence, which I think will make them both a long remembered pair of screen psychos.
Ultimately, this is a horror film. A secular horror film. This is Rosemary's Baby meets The Omen, without the comforting fact that, as there isn't really an Antichrist, none of this unpleasantness could really happen. Because he's not the anti-messiah, he's just a very naughty boy. It's a portrait of a psychopath, without the showy theatrics of a Silence of the Lambs. It's just a kid, who we can see just isn't right, and the knowledge that one day he's going to do something appalling that will destroy the lives of everyone near him, and the film delivers a series of emotional punches to the gut which don't let up, right up until the very end.