Willow was completely justified in doing all of the things she did. First of all, she was already kinda addicted to magic all through the series, and Tara was helping her work through that addiction. When Tara died, it seemed that the barriers were thrown down. Willow lost all the control that Tara had helped her gain. Going after Warren was justified, though I don't really condone the flaying, she was angry, and anger can make you to the wacky. And ending the world, all she wanted to do was make the pain go away, and sometimes, ending the world, or something equally important, such as a life, seems like the only way. So no, I don't think Willow went to far for revenge.