I see this all of the time. I don't understand?

During the final season McNulty flipped the script and began behaving in the same way that the city/department behaved.

It was purely to illustrate that if a human behaved the way that the Baltimore Police Department behaved you would view it as a self-destructive deranged person surrounded by a cast of enabling supporters with questionable morals.

I, personally, thought it was brilliant.