I know a couple of people find him boring because he doesn't have the charisma of Avon or Stringer, or even the moral complexities of those characters, but I found Marlo fascinating.
He treated everything so cut and dry, so business-like. Like in this scene, you know? "You got in my face? You'll have to die." It's a rule and it's simple to follow. But at the same time it's not business, because he enjoys it. Or at the very least, he needs it in order to function, as exemplified by his last scene of the show. Yeah, he's a psychopath. But I guess I needed that in the show after every other drug kingpin with moral underpinnings.
One of the feelings I got from him was that he's been doing this so long, killing is a mere annoyance to him now. Something so routine and bothersome that he doesn't even attend to it himself anymore. He's thoroughly terrifying in that sense. He's like a combination of Avon's ruthlessness and Stringer's patience and intelligence combined with an added sense of callousness.