Parental hatred
Instead of being a source of life and hope and opportunity, which is the role parents usually serve (in addition to many, many more of course), the parents of the shooter and his friends all have something in common: They don't love their children. The novelist is undoubtedly pointing his finger at hateful parents.
The irony of signs
Were there signs that could have been used to predict Len's outrageous behavior? Yes, of course, but the point of the interviews is to show that those red flags were buried in a long list of red flags. No one really knows what the difference is between a real school shooter, and a grumpy, angry teenager with a bad home life.
The irony of violence
Violence is implied by the premise, but it wasn't implied by real life. Len had to decide to pursue evil before hateful, murderous impulses arose within him. It wasn't necessary that Len's hatred should lead to real violence, and the idea of harming another person seems only to represent violence against any sense of reality, existence, God, love, life—all of it. By acting in violence, Len severed himself from the heart of life and became an avatar of hatred and death.
The irony of the community response
Who's to blame for Len's suffering? Is it Len himself? Is it his parents? The community scrambles to make sense of these things, but everyone already had the sense that some people were being mistreated. The question then becomes "What on earth could we have done to stop it?" The irony is that the community pulls together in the chaotic aftermath.
The ironic relationship
Instead of Len being locked in a basement, raging against some sort of tormentors, he is shown with a girlfriend. That means that Len must be at least well-adjusted enough to earn the love and loyalty of at least one girl. The relationship itself seems to be one potential reason why Len might consider love instead of hatred, but he doesn't.
