Would someone do something dramatic for the glory of Mormonism without revealing he was Mormon?
You could say that for any potential motive this guy might have had, if he left no clues to “why”. Why did he do it if he was angry at society, and not leave a message? Why did he do it if he was depressed and suicidal and didn’t leave a message? why did he do it if he were targeting a certain political movement and not leave a message?
Regardless of why this guy did it he apparently didn’t leave any kind of message or trail explaining his mindset.
THe mormon missionaries may be inconvenient a few miles from Hill Cumorah and the Sacred Grove, but they aren’t doing it down the barrel of a gun.