Missed oppoerunity. No, not what you're thinking.
This show never really addressed the fact that some people would agree with what Dexter does. The Punisher, Death Wish, Boondock Saints, The Equalizer, Batman, and many more movies, shows, comics, and books over the years have dealt with vengeance, vigilantism, and other moral and ethical issues that go with the territory.
No, I'm not talking about a single episode about The Dark Defender, but it being an overarching theme in the series. Once word of The Bay Harbor Butcher got out, there would absolutely be support for him, but it's not a consistent or big theme in the show. They teased it at times - Miguel and Lumen - for example, but it wasn't as big a theme as it should have been. Hell, Doakes himself made some questionable kills in this show and he felt very justified in doing it.
It's not believable that every single cop or person that works in law enforcement would be against Dexter. Harry saw injustices every day. Lots of cops do and there are people who would absolutely support this. I'm just surprised they never fleshed this angle out more.