"I wanted to stop the whole time, but I just couldn't. I didn't have the will to do it. And that goes against my nature," he said. "I haven't really figured out why I did it."
We think of ourselves as having control over our actions. We're pretty good as justifying most of the things we do within mental models we make of ourselves. However, the reality is that often our actions are driven primarily by external forces. The human brain is full of biases created through evolution that shape our behaviour far more we often like to admit. I think the feeling of disgust that media like this induces is primarily that realisation. We look for reasons why it can't be so.
The good news is that once you start to understand where our biases are we can compensate to a degree, or at least include them in our mental models.
Wikipedia has a nice collection of cognitive biases http://en.wikipedia.org/wiki/List_of_cognitive_biases and http://lesswrong.com/ is an interesting community trying to educate itself on this topic amongst other things.