If one in every thousand Americans has a unique and unexplained mental illness that compels them to violence, that would provide more than enough explanation for motives. It's even true for some famous shooters. [1],[2],[3]

1. https://news.yahoo.com/psychiatrist-colorado-movie-gunman-th... 2. https://www.cnn.com/2013/11/25/justice/sandy-hook-shooting-r... 3. https://trove.nla.gov.au/newspaper/article/107885790