Many see Marlo as the most "evil" being on The Wire. He lacks the humour and profane wisdom of the more popular B'more denizens, but I believe he is key to understanding the show's themes. He is, in effect, the ultimate bureaucrat, one who plays the system without empathy or fear. His sole aim is the increase of bureaucratic power – both his own power within the bureaucracy, and the power of the bureaucracy itself. Although he is certainly a person without a moral backbone, it is not really a question of good and evil at all, but of efficient success. Marlo's methods are the same approach taken by (say) Rawls; the only difference is the bureaucracy is the drug distribution system, so the brutality is more overt. The system itself is the evil, Marlo is just another player, albeit more successful than most.
The Wire re-up: In defense of Marlo -- Ben Davie