The show seems to be comparing different justifications for murder - legal, paralegal, for an internal moral code (Angel and Deb), for revenge or an externally imposed moral code like Dexter, or for Id-pleasure like the pack. Dexter at one point tries to say that he's doing these murders to protect innocents, but that's not his primary reason, although I will say it is an increasingly stronger secondary reason

Indeed! It's a fascinating sight. I tend to think that, unlike more classical escapist fantasy fare - think The Vampire Diaries - Dexter is not at its best when it humanises murderous protagonists, but here, in this storyline, I've felt differently for the first time.