I thought it made perfect sense.
Happy go-lucky magic-boy is suddenly exposed to heretofore-unseen phsychological suffering (what with every soul that he had loved being being burnt alive before his very eyes, not to mention his own subsequent cremation), which naturally results in his mind breaking beyond any hope of repair. Divine spark allows magical boy to return to (un)life as a charred corpse.
Now, if you've watched many horror films, the following equation should make sense to you, and will explain the Torchbearer's resulting actions in their entirety...
Insane + Undead = GRUESOME, INEXPLICABLE, MASS MURDER!! 