It's getting harder and harder to explain why this guy is seemingly just immune to pain all this time, without any visible influence of drugs, magic or superpowers.
He's not even supposed to be able to move at all while being electrically shocked, so I'll just chalk it up to fiction, I guess. Maybe Maria's chainsaw can open him up nicely so we can better see why. Nanomachines, son
But anyway, I guess the point of his abnormal toughness is to make him seem more menacing and almost superhuman to make Maria prevailing over him seem impressive. Like a boss monster in a video game with long and/or multiple life bars to match.