I'll need to look again at the movie to pick up any finer points, but my general impression is one of agreement with Dane.
Seems to me like he has become totally institutionalised, and obsessed by beating the prisoners. At the end he is reassuring himself that they drowned, clinging to the hope that they did, otherwise he is beaten. In a way, he's beaten anyway, because of the uncertainty. The nagging doubt probably dogged him for the rest of his life.
He knows these are resourceful, and in many cases, intelligent prisoners. I wonder if he hasn't got some kind of a chip on his shoulder. Maybe he wasn't the brightest cookie at school, or his parents had high expectations that he didn't feel he met. Sorry if I'm sounding like a social worker, but people often want to control other people to disguise their own inadequacies, and to reassure themselves that they are worthy. By preventing the prisoners escaping, he was proving to himself that he could do better than them. It was more than a job, it was personal.
He would have been humiliated at the end. The policeman tells him that he is wanted on the next plane to Washington, presumably to account for himself. He can't have relished that prospect. (Incidentally, when I first heard that my reaction was that it seemed a bit unrealistic, and over-the-top, that he'd be called to Washington so immediately. But maybe I'm way off there.) Again that reinforces my impression of someone wanting to be worthy - the teacher is going to tell him off, and he wants to be the good boy who gets the praise.