Star Wars: Clone Wars Season 7 Episode 11 - 5 Ups & 2 Downs From "Shattered"
3. Up - The Clones Turn
"The time has come. Execute Order 66."
When the clones tell Rex he's been called for a briefing, we know exactly what's about to happen. Emperor Palpatine, voiced by his original actor Ian McDiarmid, repeats his iconic line over a hologram. With those words, Rex stops being Rex, and becomes Palpatine's greatest weapon.
He and the rest of the clones lose themselves, and surround Ashoka, drawing their blasters. The scene is intense, especially after Ashoka's praising of the clones as soldiers and Rex as a friend. As they prepare to fire, Ashoka whips out her lightsabers and defends herself, eventually having to attack some of her former allies, before escaping.
The scene is absolutely heartbreaking, more so than the version in Revenge of the Sith. In the film, the Jedi who the clones kill are underdeveloped, more set pieces than characters. In the show, however, Ashoka, Rex, and many clones we don't see here are fleshed out characters, so watching them turn on each other hurts more.