DISCLAIMER: I think this is easier to understand if you've seen THE WIRE. But I'll let you be the judge of that. It may contain minor spoilers, but I think I remained vague.
Marlo is a clear-cut antagonist in a world where nothing is supposed to be black or white. If you've followed THE WIRE, you might have felt for Avon and Stringer, the two previous drug lords of West Baltimore. Especially during this scene, after Stringer showered his old friend with gifts, the reminded each other how far they have come. According to THE WIRE'S logic, they were humans with hopes and dreams like everybody else, only they were born on the wrong side of the law. Marlo couldn't be anymore different.
First of all, look at him. He looks like a sad puppy. There is nothing despicable about his image. Most antagonists sell the punch before opening their mouth. Javier Bardem's Anton Chirgurh was awesome, but reason why this article isn't about him is that you knew he was fucking mental from frame one. Bad guys usually have "the evil eye" or some twisted physical characteristic that is the equivalent of wearing a sign around their neck. Sometimes it's just a very bulky frame that symbolizes how big an obstacle her is to the protagonist. James Bond villains are particularly bad at this, but it became somewhat a calling card of the franchise over time. Marlo's a wolf in sheep clothing. You would sit next to him on the bus and not notice him.
So, what did you like about Marlo Stanfield? What other antagonists did you love?