Dexter's raison d'etre for being a serial killer is that he "kills people that the police can't bring to justice". But in this episode that just wasn't the case.
I as understood it the car killer got off because his lawyer said that he had completely got over his drinking problems and therefore wasn't drunk when he hit the young lad. But Dexter very quickly found Matt Chambers drinking in a local bar. Then he easily discovered that he similar drinking misdemeanors in other states. Once that proof had been uncovered it could very easily have been given to the police to bring him to justice. In this case it didn't really give Dexter a proper justification for killing him.
Sloppy writing really which really detracted from the remainder of the episode. To me it was a bit like finding that the Mona Lisa had a cauliflower ear. A small error which upset the whole picture.
The series does have great possibilities. The characters are beginning to develop but it does need sharper writing if it is going to keep my attention.