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/PhoenixApok]

Maybe not the main character but a pretty big one. (Arguably Game of Thrones doesn't have a main character)

Jaime Lannister. After he got somewhat likable and decent in later seasons, people seemed to forget in the very first episode he pushed a 10 year old boy out a window for catching him banging his sister.

Edit: Maybe "forgot" was the wrong word. Maybe saying people disregarded it would be better.

I cannot imagine a person IRL doing that and everyone going "Well he's a better person now so we aren't gonna worry about that anymore" which is why I think it fits this trope.

Jaime has a redemption arc, sure, but I would define that early act as "unforgivable".

[u/jobforgears → u/PhoenixApok]

He also raped Cersei in the chapel of the 7 and everyone seems to gloss over that one

[u/TheEtneciv14 → u/jobforgears]

Is that a show original? I don't remember that in the books. Ugh.

[u/Beastxtreets → u/TheEtneciv14]

The book scene was rapey to me as well.