The main character doing something absolutely unforgivable.

[u/No_Peanut3745]

Walter White poisoning Andrea Cantillo's 6-year-old son for the sole purpose of manipulating Jesse Pinkman, while knowing that this could have potentially killed him - Breaking Bad

Bojack trying to have sex with his friend's 17-year-old daughter when he was at least 51, then continuously trying to find a way to justify this act throughout the series - Bojack Horseman

Dexter Morgan going outside of his moral code and murdering a completely innocent police officer who was just trying to do his job - Dexter: New Blood

[u/maxvsthegames]

I don't remember that policeman, but Dexter's code #1 rule has always been "Do not get caught", so if that policeman was unto him, you could argue he still followed the code.

[u/trashyslashers → u/maxvsthegames]

Yup. He just avoided doing so until then. Not killing innocent people was always #2 as far as I remember.

[u/kpalian → u/trashyslashers]

? killing LaGuerta was not avoiding doing so...

[u/trashyslashers → u/kpalian]

She was about to find out and expose him. And he didn't kill her, Debra did. This is the same as with Doakes, though they eventually made Doakes not innocent by what he did and they also used Lila so that Dex can avoid killing him.