Gaping plot hole (SPOILERS)

How on earth could a middle-aged man who's been imprisoned for 30 years in a cellar, doped up regularly, and fed on scraps, have the strength, skill, and hunting instincts to butcher all those people, including battle-hardened soldiers, so easily? In the end, this isn't a realistic horror tale, but a dark fable rooted more in Greek mythology. I mean, the two sisters could have arranged some sort of accident that would handicap Steven, and that would have exempted him from military service.