Thinking about his armor set description a new idea began to emerge. One piece of the description reads "During his solitude, he forsook everything, for he believed in the goddess's love for him." The interesting part is that they use the past tense of "believe". Could it be that Lautrec no longer feels this way? There may have been a falling out between Lautrec and Fina; she may have wronged him in some way and in his anger he decided to take revenge upon her by destroying her very way of life. This would explain why he kills fire keepers but never uses them, why he left the black eyed orb at the fire keepers dead body to lure undead to him and harvest their humanity.
Another possibility is that the gods had turned on Fina and possibly killed her and Lautrec is actually taking vengeance on them in her name.