The narrator seems to view Pechorin as intriguing, but ultimately a bit off-putting and distasteful. I would guess that he finds Pechorin to be as heartless as I believe him to be. On the other hand, Maxim viewed Pechorin as a close friend, and thus overlooked his faults. Thus, he took Pechorin's behavior as a friend's betrayal. He did not notice Pechorin's faults, and so Pechorin's 'betrayal' made no sense to him.