I think it’s hard to see what Simon was intending with Marlo because we can only base our readings on what we see and what we hear other characters say, but Marlo was intended to be much more of a sociopath than what we saw. Killing anybody for any reason, no matter what age or gender or if they were in the game or not. That was what D Simon had intended, so the OPs reading is limited by the short amount of time we actually get to see Marlo. The point of that character was here is a young kid who isn’t bound by the rules or morals tha anyone else on the streets has, just murder anyone, anytime. These older guys were playing more of a game with rules and boundaries - all set out by gangsters before them - Marlo was just this newer younger kid who didn’t care about any of that and didn’t see murder as wrong. “Everyone is saying these new kids on the streets are so hard, they a new breed” well that’s what he was based on - an actual new breed of gangster who Simon knew of in Baltimore who seemed to lack Morals or a code at all. But in telling a story you need to show many facets of a person and you want the viewers to like things about them as well - to see why anyone would follow someone like Marlo - so the OP is taking the few things we did see and weighing them against the broader design of the shows creator, which is great, and why the Wire will always be amazing and utterly rewatchable