After Anakin lost his duel against Obi-Wan, the Jedi simply grabbed Anakins lightsaber and left, presumably because he couldnt do anything else to his former apprentice. However, Obi Wan had the perfect opportunity to kill Anakin and prevent the continued rise of the Empire. The Sith was, for all intents and purposes, defeated, but as long as he lived, the Emperor would find a way to keep Skywalker as his servant. While it was true that Obi Wan considered Anakin his brother, he could have killed Anakin out of mercy to end his obvious suffering. Instead, Obi Wan let Anakin suffer even further, and once Vader was reborn, he, alongside the Emperor, brought a new wave of tyranny to the Galaxy. Though this obviously set in motion the events of the original films, it demonstrated an act of stupidity on Obi-Wan's part.