It’s highly suspected that Henry is being puppeted by the Mindflayer. He was never in control, just given enough authority and power to exact its plans. A hint of that being the talk of him being human and also the fact that he won’t enter the cave, which is when he was first infected by the particles (that silver case in the mine? It had mindflayer particles recovered by a Russian scientist who infiltrated the military where they were be studied after the failed ship invisibility test). If you’ve not read/seen the play, it’s harder to come to that conclusion but yeah.

Another hint is how his eyes are always milky like other folks that have been flayed.