Well, I can understand your desire for a little more light being shed on Dante's past, but the problem is that this is not the scope of the intro. The intro just aims at presenting Dante as he is is now. Like saying "Ok, this is the man you're going to deal with for this story. This is the situation now." The reasons why he ended up like this can be explained later in the game, maybe with flashbacks like the one you suggested.