Responding to the question of how Dexter can be admiring the scumbucket's lying and then suddenly get so enraged that he kills him:
D. admires the ease the scum seems to have lying. Lying is obviously important to D.'s survival in general. But specifically he has had trouble lying to Rita lately--this gets him sent to the NA meetings(which he doesn't want) and then into the doghouse with Rita for not wanting to continue to go to them. So D. is listening to the scum's relentless, glib lying and wishes he could have done better himself lying to Rita.
But when the scum's attempts to connect with D. so as to save himself lead to discussion of women the ugliness of the scum's attitude toward them surfaces and he calls D.'s GF derogatory names in a venemous outburst that triggers D. to attack him. Yes,D. has previously shown emotion. But here he realizes unexpectedly his emotion for Rita specifically--that she is not "just a companion" as he has just described her--trying in vain to dismiss her importance as he is threatened presently with losing his relationship with her because of his refusal to go to NA further. He attacks the scum at that moment because the scum's words are so insulting towards this person that D. suddenly realizes he does now care about despite that he apparently did initiate the relationship more with the goal of simply projecting normalcy so as to keep suspicion further from him.
So I agree that this seems to be the first time we see emotion about Rita from Dexter. He realizes he does actually care about her as a person' not just as a means of being more inconspicuous.

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