Star Wars: Episode III – Revenge Of The Sith (2005)
The Betrayal: With his morals twisted by evil Chancellor Palpatine (Ian McDiarmid), Anakin (Hayden Christensen) does the unthinkable and slays innocent younglings - nothing says "screw you, Jedi Order!" like a massacre.
Why It’s So Bad: He killed children! There’s no coming back from that…
The Fallout: Just Episodes IV-VI, y’know, as Darth Vader becomes a force to be reckoned with.