You realise the kid probably also… (spoilers)
…killed his father, as well. You know, to keep his mother ‘happy and safe’. Seeing how he went about killing Phil without anyone suspecting anything, and how he mentioned that he was the one who found his late father hanging (and after drinking up until the very end).
The facts start lining up if you think about it. Him bringing paper flowers to his father’s grave simbolizing his artificial regret. He did prove to be a capable psychopath, so he probably did it.