This is based on opinion. Please don't list it on a work's trope example list.
Tear Jerker / Forsaken Chronicle
Connor Omega's ending in the first game. Sure he kills Gage and saves the world from his evil plot. Then Exie runs up to him to congratulate him...only to be repeatedly beaten and stangled. Then Connor comes to his senses and reverts to his normal form...just soon enough to see what he's done.
Get's worse in the second game, as throughout the story Connor Omega has been killing EVERYONE he loved or cared about. Come the ending he regains control only to realise that he has nothing left. His no input ending has him lose control again...
In Ultra Exie appears in a heaven-like stage with a slight echo in her voice. It is implied that since Connor never doubted Gage in that timeline, he was completely loyal at this point. It is heavily implied that Connor killed her during their fight...