@Theaetetus,
Yeah, I just see no reason for the mugger to stab the character.
As for how the mugging would influence the character, it's hard to say without having the preceding events. Though, if the stabbing can be smoothed out, that direction seems plausible.
In my own writing, the underbelly of society is top interest, so this sort of story is attractive. The narrator, the character who is stabbed, seems to have a sort of distant, WASP kinda view of the whole thing. Which I guess would make sense with the planned turn of mentality into dark and cynical after such a visceral encounter with a slice of that reality. I dig the direction.