there's an interesting dynamic with Frank where he is corrupt/chaotic AF dealing outside of the union, but is lawful or adherent to the values and tenets of the union. Which to him includes fucking with the cops, doing shady deals to advance the union interests, even doing a little shady within the politics of the union so that he can finish doing his good union dirt, etc. But I think it is highly consistent for his character to be making Nicky and Ziggy sink or swim.
Probably a tighter and more accurate way to describe my thoughts. You went ahead and disagreed with yourself but I'm going to jump on board with your disagreement about the morally ambiguous characters - you would have a way shorter list if you named the morally unambiguous characters from the show, cop or otherwise. And on that list, I think you'd only have morally negative characters and probably one of them would be Rawls? Rawls and Marlo? Does Snoop have any good to her? Wee Bey and Chris Partlow are some of the most brutal characters on the show and they are demonstrated as having some kind of good. Season 1 Episode 1 Scene 1 of the show is the crime scene where D murdered Snot Boogie.
Actually really the thing where I think I'm disagreeing with your premise is that Frank is even painted as a "bad guy". Although even with that..... is Avon even actually a "bad guy"? Or Stringer, in seasons 1 and 2? For sure they are criming and murdering, but it's kinda within the rules of their world and maybe that's a big part of the show. The Sopranos juxtaposed two sides of Tony - he's a crime lord out there, he's a suburban family man in here. A big part of what makes The Wire great is that it takes that concept deeper - these are street junkies and criminals and they are also humans dealing with the same kinds of problems every human does.