More likely is that your inaccurate map of the territory of his mind was sufficiently wrong that it fell under the “normal person” category. As someone who has fantasies about that sort of thing (but would hopefully never actually do it), let me tell you that this isn’t the sort of thing that comes out of nowhere. Odds are, he knew where his proclivities lay, and simply decided not to actualise his fantasies until alcohol reduced his inhibitions sufficiently that he decided to go through with them.
More likely is that your inaccurate map of the territory of his mind was sufficiently wrong that it fell under the “normal person” category. As someone who has fantasies about that sort of thing (but would hopefully never actually do it), let me tell you that this isn’t the sort of thing that comes out of nowhere. Odds are, he knew where his proclivities lay, and simply decided not to actualise his fantasies until alcohol reduced his inhibitions sufficiently that he decided to go through with them.