Here is Thomas Hawk on the 2007 Pulitzer Prize-Feature Photography. I cannot come up with a better description than the one on his blog.
A broader question is why do we watch tragedies? Up to an extent, having our emotions moved reminds us that we are human. I wanted to see if any of the victims at VT went to my high school, so I looked them up on facebook. It was eerie seeing the random pictures people put up on facebook as their portraits. The profile pages were left behind. Previously, the events had been less real, more remote.