He was deceived by a lie. We all were.
4 seconds sound clip from the Star Wars: Episode III - Revenge of the Sith movie soundboard.
You can hear this line at 01:39:24 in the Blu-ray version of the movie.
Quote context
[...]
- Padme. I need your help. He's in grave danger.
- Padme, Anakin has turned to the dark side.
- You're wrong. How could you even say that?
- I've seen a security hologram of him killing younglings.
- He was deceived by a lie. We all were.
- It appears that the chancellor is behind everything, including the war.
- I don't believe you! I can't.
- You're going to kill him, aren't you?
[...]