Grief over Tara’s murder completely overwhelmed Willow who went on a magic bender for the ages. It culminated with her need for vengeance driving her to not only kill Warren but flay him alive. It seemed that no one could stop her magic rampage, not even the Slayer.
So when the Buffy the Vampire Slayer season 6 finale opened, we really believed that Giles would be the person to get evil, angry Willow to see the light.
Boy, were we ever wrong, and the results couldn’t have been more satisfying…
Willow: Uh oh. Daddy's home...I'm in wicked trouble now.
Giles: You've no idea.
I’ve always loved Willow, but dark, sarcastic, drunk on power Willow was like nothing we’d ever seen! That sweet, smart, shy girl we’d met at the beginning of the series was nowhere in sight, but that’s exactly the Willow that Xander tried to reach as she stood on the verge of obliterating the world.
Yes, we know that the pure magic that Willow stole from Giles helped dilute the power she had gotten from her rage, but it wasn’t magic or superpowers that pulled Willow back from the edge, it was the love of her best friend who wouldn't give up.
Xander: First day of kindergarten you cried 'cause you broke the yellow crayon and you were too afraid to tell anyone. You've come pretty far, ending the world, not a terrific notion but the thing is, yeah. I love you. I loved crayon-breaky Willow and I love scary veiny Willow. So if I'm going out, it's here. You wanna kill the world, you start with me. I've earned that. – Christine O.