Maybe drug abuse? He was scheduled for a random drug test the day of the shootings.
Please read Sisku Hanne's post at 1720 about a recent incident in their area. I have heard twice now (different media sources) about the killer's mood having 'darkened' which I find to be an odd choice of wording. Was he ususally moody and just got more so, or did he have some kind of breakdown? No explanations given yet...but I am sure more will come out as time passes.