While each Star Wars movie has its dark moments, Revenge of the Sith is arguably the darkest chapter in the saga, and Order 66 played a large part with that… although seeing a man lose nearly all his limbs and be horribly burned on a lava planet brought it home.
Although a scattering of Jedi managed to survive Order 66, Obi-Wan Kenobi and Yoda among them, the newly-titled Emperor Palpatine was nonetheless successful in not just destroying the Jedi Order, but also painting the knights as traitors to the public.
Most of the Order 66 sequence in Revenge of the Sith showed the clone troopers eliminating their former commanders, and all we saw of Anakin… sorry, Darth Vader’s part in the devastation was him igniting his lightsaber in the High Council Chamber where numerous younglings were hiding and a hologram recording showing him dueling Cin Drallig and his apprentices.