Are we the bad guys? – Books where the main or even a major side character is disgusted by the actions of his side and sabotages/turns against them.

Or he is appalled by both sides or even his entire species and decides to burn it all down. I remember watching a Da Vinci Code movie about the villain using a bioweapon to kill off half of humanity to stop overpopulation years ago and thought that was an interesting plot.

Genres could include thriller, sci-fi, historical fiction or fantasy (although no OTT or dominant magic such as resurrection please). True stories are welcome as well. Particularly keen on books with characters that have strong morals but will break the rules to do what is right e.g. Jack Reacher.

Thanks in advance!