Warren shot Buffy. Warren shot Tara. Buffy's alive, Tara is dead. Willow found out, and, being the most powerful wicca in the western hemisphere she decided to get the payback, with interest.
Think your little witch buddy's gonna stop with us? You saw her! She's a truck-driving magic mama! And we've got maybe seconds before Darth Rosenberg grinds everybody into Jawa burgers! And not one of you bunch has the midi-clorians to stop her!
Let me tell you something about Willow. She's a loser. Always has been. She got picked on through junior high, high school, right up until college. With her stupid mousy ways. And now? [laughs bitterly] Willow's a junkie. The only thing Willow was ever good for...the only thing I ever had going for me were those moments...just moments...where Tara would look at me and I was wonderful. And that will never happen again.
[to Buffy] Please! This is your pitch? You hate it here as much as I do! I'm just more honest about it. You're trying to sell me on the world? The one where you lie to your friends when you're not trying to kill them? Where you screw a vampire just to feel and insane asylums are the comfy alternative? This world? Buffy, it's me! I know you were happier when you were in the ground. The only time you were ever at peace in your whole life was when you were dead. Until Willow brought you back. You know, with magic?