Combined with the other comments, I think you've hit an essential piece of info. The Death card is not about death but about terminating a particular phase in life. The card and the casing might have been his goodbye note.
Other reports had something written on the back like "Father/Mother/Son/King/Priest". Which would all appear in a Tarot casting. He may be that whacked that this was his goal to reach "Godliness". drop 5 that fit the requirements.
The high school comments are also appropriate based on the shooting times. All before and after school hours.
Profilers are probably working overtime on this. The stilted language, the hours, the hits and the goodbye clues.