This episode is a perfect demonstration of Walter White changing to heisenberg. His obsession with the fly or the 'contamination' reflects his charachter in one of the best ways the shows has done up to this point.
Many reviews say this is a filler episode. It is not. He confesses to Jesse that he killed his girlfriend. The moment he kills the fly is a metaphor for him finaly getting rid of walter white. In season 4 and 5 he's almost constantly heisenberg and we can hardly see his soft side.
