Remembering Kevin

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.