I don't know if I've posted this before/someone's mentioned this oops, sorry if I've had/took someone's idea but don't ya guys think that the black flames are affecting Shima psychologically? I mean, assuming he's had Yamantaka since he was young, and that the flames can burn out a demon's soul, how could it not affect him? We know it affects him physically makes him pass out, thank god so what's to say that the flames aren't partly the cause of his 'rebellion' against his family/good and general personality? That it's a power he can't control fully and is affecting him both psychologically and physically.
regardless, this is a headcanon of mine until proven otherwise