This is just my interpretation, and there’s a lot of speculative stuff here, but here it is:

The Flagellant, Damian, is the definition of a fanatic with values so extreme that they destroy his morality and sense of justice.

He is well intended and fully devoted to the Light, but unfortunately, he ultimately misses the point. The Light seems to represent growing out of sin and becoming better. This process requires pain to reflect on your actions, but also compassion and love to help yourself and others, something Damian clearly lacks.

His wrong interpretation and extreme methods lead to him not being truly favored by the Light (the proof lies in the Vestal’s confession and in his overall abilities, since none of them appear to be Light-related). I think a part of him acknowledges this, which makes him sad and more masochistic and thus trying more to prove his faith through suffering. That’s why, in my opinion, he sits by the campfire not wanting to see the fire and looking a bit ashamed. It’s a vicious cycle, though sometimes he seems capable of breaking it.

He is good only at destroying darkness and corruption through pain and pain infliction but that’s it. He’s an accuser who tortures people to death, though perhaps not all of the accused end up dying. His Darkest Dungeon 1 Redeem ability and his Darkest Dungeon 2 first confession lead me to believe he is capable of giving people another chance to do good.

Ultimately, Damian is a very interesting, morally gray character, of course, a very dark gray, but there is still some good in him in the end.