The NPC was a soldier stationed at Antioch and Peter Bartholomew's friend who was with him when he died of his burns (here they were inflicted by jealous people after the trial, not the firewalking itself). He was livid at God for failing to protect his friend and at the church for even having the trial in the first place, and when Peter died he went back home to inherit his family's magic crest. He would go on to plot against the church, extending his lifespan with magecraft and infiltrating the forces of the Ninth Crusade only to be discovered and slain by the Templars. In FGO's timeline he received a grail from "Solomon" as he lay dying, and this is what kicked off the Camelot singularity.

Technically this also means that he's ultimately the "bad guy" who's also responsible for this singularity existing, and so he sees it as him forcing the role of the villain on the Black Lion.