Dean's self-image being what it is, I wonder if he doesn't think he kind of deserves hell for "cheating" that guy out of life in Faith and for failing his Dad and failing Sam, etc. I think that might have a large part to do with his actions because there's also no way he could articulate that to Sam. Because Sam would argue that it isn't true. But I think Dean feels his "failure" as a person and his thievery of life keenly.