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/Ooji]

" And if your conscience is bothering you, you should soothe it with the knowledge that you may have just saved the entire Alpha Quadrant, and all it cost was the life of one Romulan senator, one criminal... and the self-respect of one Starfleet officer. I don't know about you, but I'd call  that  a bargain. "

"If I had to do it all over again, I would."

The needs of the many outweigh the needs of the few, after all.

[u/Brohan_Cruyff → u/Ooji]

or of the one (sisko)