With Willows thoughts being beyond all logic/reason, Willow fights Buffy: eventually rendering Buffy unconscious (by hitting her with some powerful blue lightening). Fate then intervenes – as Giles returns, leading to a magical duel between himself and Willow: he blasts her across the room, he commands her to stay down and he binds her in a stasis field (to prevent her from causing further harm).
Unfortunately, Evil Willow is too powerful to be defeated so easily: she’s able to use mind control (on Anya) to help free herself from the stasis field. The magical duel then resumes, with Willow eventually defeating Giles and absorbing all his powers (which he borrowed from a coven). In doing so, Willow becomes more powerful, which combined with her emotional state, leads her to conclude that: the whole world is suffering emotional pain (and that it must be stopped). Thus, she decides to destroy the world (even though this goes against everything that Tara stood for). Fortunately, Willow has managed to overlook one emotion: love. And it is love that finally defeats her: when Xander tells her he loves her (as a friend).