Fired from the Council, Giles remained as a member of the Scooby Gang for two more years, trying to find his way even after being reinstated in his position as Watcher. He left Sunnydale in 2001 when he came to the realization that he had to leave Buffy so she would embrace the responsibilities of adulthood. He returned to Sunnydale the next year to stop Willow Rosenberg from destroying the world and nearly lost his life in the process. After she was stopped, Giles took her to England to aid her in her rehabilitation and coming to terms with her power.
When the First Evil began its campaign and had the Watchers Council destroyed, Giles gathered a group of Potential Slayers and returned to Sunnydale to prepare for the coming battle. However, he came into conflict with Buffy regarding the vampire Spike, whom Giles considered a potential threat. When Buffy's leadership was questioned by her friends and allies, Giles sided against Buffy but reconciled with her when Buffy devised a plan that would allow the Scoobies to defeat the First. When the power of the Slayer was activated in all potential Slayers throughout the world, Giles helped Buffy create the Slayer Organization, though in secret he sent Slayer Faith Lehane to eliminate a rogue Slayer. After Buffy discovered Giles had gone behind her back, they were not on speaking terms for a while, and Giles joined Faith as the two sought troubled new Slayers to help them come to terms with their powers and prevent them from falling into the path Faith had fallen to in the past. A short time later however, as the Twilight crisis came to the near destruction of the world, Giles was murdered by Angel, who was possessed by the sentience of a newly created universe.
Source: Buffy Wiki