Why not simply say that The Spirit moved him like it did Nephi? He could phrase it somewhat like this:
"I was constrained by the Spirit that I should burn the churches; but I said in my heart: Never at any time have I torched a building. And I shrunk and would that I might not burn them. And the Spirit said unto me again: Behold the Lord hath delivered these buildings into thy hands. And now, when I had heard these words, I remembered the words of the mormons which they spake about me in the church. Therefore I did obey the voice of the Spirit, and took a match and lit it, and did set fire to the buildings."
If they don't accept it and drop the charges, that would be an acknowledgment that Nephi, that great prophet, would have been arrested and arraigned in Utah on murder charges, and possibly be evaluated on his mental health.
True, but Utah law enforcement wouldn't know or care. All they would care about was nabbing the perp going around beheading people.
Updating to a modern scenario -
St. George police find a prominent wealthy mormon, LaBan Christensen, headless and naked in the street. (His blood alcohol content is three times the level for DUI purposes, but this detail is never released to the public. Can't have a scandal.)
Soon after, the Utah state police announce that a suspect has been captured. He was covered in LaBan's blood, was wearing LaBan's clothing, and carried a bloody machete. His name is Niephyy Sorensen and happens to be the son of a St. George stake president, Lee Hy Sorensen.
Even Lee Hy, with all his connections, wouldn't be able to spring Niephyy; after all, LaBan's family would be locally powerful as well. Niephyy's "the Spirit made me" defense wouldn't hold up in any court, not even in Utah.
"Prophetic purpose" only works with 2600 years of hindsight. If a modern-day Nephi was loose on the streets, Utah mormons would be terrified and demand he be locked up and eventually executed.