Dexter never pretends that he's acting from a moral impulse or what he's doing is particularly righteous. He's a creepy serial killer and self-confessed monster who gets off on killing people and his framework for only killing people who 'deserve it' is one taught to him by his stepfather.
That's a careful balance. On the one hand Dexter murders people the audience is rarely invited to feel sympathy for, but on the other hand he never tries to insist to us that his motives have a moral impulse. He's a vigilante without the sanctimoniousness of say, most superheroes.
1. I don't accept the right of the state to execute criminals. I oppose the death penalty and strongly favor is abolition.
But most of us accept under certain conditions the wholesale slaughter of entirely innocent adults and children. The so-called 'collateral damage' in warfare. Our attitudes to violence revolve around lines of acceptable and unacceptable uses of bloodshed.
Dexter is behaving in a manner most would consider unacceptable, but doing it in a way that contrasts his behaviour versus 'worse' examples of his chosen profession.
In that respect he's like a lot of cable TV protagonists - explicitly engaged in unsanctioned behaviour (polygamy in Big Love, the mafia in Sopranos, crystal meth in Breaking Bad), but still looking morally better than 'the competition', who usually engage in something similar but are nastier about it.