… because he was stopped in the middle of the process of attempting (trying) to take a human life illegally - by his own written admission and actions that day. He was attempting to murder someone. He had picked a target, gathered intelligence, arrived with otherwise legal weaponry, written a manifesto, concealed himself and in all likelihood would have taken the shot when the opportunity presented itself. Whether he got to pull the trigger or not is immaterial - he has already begun attempting to kill someone by setting his plan in motion.