Witchy Willow Backfires
As Willow becomes more and more into magic, she often sees it as an easy way to solve problems, problems that could actually be solved without it. That's probably one of the main reasons it got out of hand. Giles warns her from the beginning that she should be careful, that she shouldn't get into it without guidance, for he knows by experience how wrong things can go.
In 'Lovers Walk' Willow plans to perform a delusting spell to put an end to her little affair with Xander, only doesn't complete because Spike kidnaps them.
When Oz cheats on her with Veruca, Willow plans to cast a vengeance spell on them, but doesn't have the guts to pull it through. Several weeks later, still mourning his departure, she casts another spell to have her will done and tries to heal the pain. The spell backfires, but her desire for vengeance and wreckless use of her power gets her a job as a vengeance demon, which she declines, but foresees her dark future.
As Tara starts to get worried, they end up arguing and instead of talking it over, Willow chooses to perform a spell to make tara forget their fight. Tara finds out and is even more upset. To solve things and make Buffy forget she was ripped away from heaven, Willow cast a stronger forgetting spell, that backfires too.
She soon falls into a crazy whirlwind and magic becomes her drug. When she looses Tara and almost looses her friends, Willow realises that magic is destroying her and promises to no longer use it. She manages quite well until Tara dies, accidently shot by Warren. Willow tries to bring her back, but fails. Her eyes go black and she looses it. She ends up at the Magic Box and sucks up several Black Arts books. Their powers take over and her hair goes black. As she sets out to find Warren, the gang figures out what's going on and try to catch up with her. Too late, as Willow flaws Warren and kills him, she then sets out after Andrew and Jonathan, currently in jail.
The gang manages to get them away as Willow chases them. She soon ends up fighting Buffy and Anya at the Magic Box. They aren't really a match for her, but luckily Giles arrives, doped up on lent powers. But Willow eventually takes him down and sets off to end the world.
Xander is the one that manages to stop her by claiming his love her her. She collapses in tears, back to normal. Giles then takes her to England to help her get over the pain and guilt of taking a life. She also leans how to control her powers properly.