He was pyschotic and alienated from his Evangelical Protestant Christianity and decided to make a statement by becoming Mormon. The fact that he (allegedly) showed up with a beard and skullcap makes me think he may have been considering Islam, too. I think he was simply looking for something that would make his rejection of orthodox Christianity clear, and chose Mormonism because it was convenient and easy to do.
Basically, however, he was psychotic. Many of the Muslim converts who have killed people here have also been psychotic. However, they kill in the name of Islam, and this guy was not killing in the name of Mormonism.
He was a nut. No other word for it. He just focused on churches than than a day care or school. He probably became a Mormon because Pentacostals have traditionally had a disdain for Mormons although that is changing in the new Emerging Church model.
The guy was a TROP convert, just about guarantee it.