Michael did that shit deliberately. I think that's kind of the point of The Wire. You have choices. Michael was not forced into being a killer. He sought out the people who killed and let himself fall into that group. Some have good luck (Namond), and while others have extremely bad luck (Randy), there still exists free will in that urban jungle and Michael, despite all pleas and warnings became a killer deliberately.
I think there's a fine line between free will and the powers that be shitting on everything in The Wire. People are held down but they still have their free will.
We had this awesome discussion about this in The Wire thread at SA when Felicia Pearson was arrested. "She's just a product of her environment. It's all she knows!" "That doesn't change the fact she was still around people like that, she should have known better" Fun times, especially when applied to The Wire.