Watch the scene again. Dexter is tied up, he kicks Liddy in the face, and then Liddy pulls out a knife and tries to kill Dexter first. Dexter turns the tables and kills Liddy.  Liddy escalated to lethal force, therefore Dexter could be argued to have killed in self-defense. Its convenient, of course, because if Liddy had not pulled the knife and tried to kill Dexter, Dexter still would have had to kill him or else he would have been caught. But at the end of the day, Dexter is a serial killer. His #1 rule is dont get caught. If you put a serial killers back against the wall, they will be left with no choice but to try to kill their way out of it. Dexter was absolutely going to kill Laguerta, for instance, which to me is one of the craziest and most egregious things Dexter has considered doing. And even though he didnt pull the trigger, it was his desire to kill her that got her killed, so I still consider her death on his hands.