Roy Mustang's revenge on Pride/Fuhrer Bradley. We learn of Roy's promise to Hughes that he follow the right path to be a great
man (meaning no revenge, in a scene where Edward is all for it). However, what does Roy do? He avenges his best friend's death by killing Bradley for revenge! He succeeds in his plan, but at the cost of an innocent child's life. Evidently, Selim Bradley's devotion to his father was something Roy didn't consider in his plan. The guy definitely had too much 'grief=rage=revenge' on the brain (and this is not some Anime1 OOC nonsense: the manga
's Roy was also too easily drawn to revenge against the murderer of his friend).
However, playing Devil's advocate, What choice did Roy have?
His earlier ambition of rising up the ranks was now a pipe dream. The list of allies with enough power to take down a Homunculus was thin: Izumi was coughing up blood. Armstrong had to protect Roy's besieged subordinates. Edward had to deal with rescuing his brother from Dante. So Bradley was Roy's job. Besides, the clock was ticking on the invasion of Drachma, which Bradley assigned Roy to commence. Now that he knew the reasons for the wars, Roy could not let another Ishbal Massacre take place.
So what we have here is a 'darned if you do/darned if you don't' situation. Bradley had to go, and his destruction ended the Homunculus' control on the country. However, with the effort Roy put in trying to redeem himself for past deeds, to end up committing selfish revenge- and to have that path lead to a child's death- must have been a heavy blow to the man (hence his situation in Shamballa
). One more mistake he will always regret, for all the good it did.