To help exact her revenge, Willow decides she needs more power, and as such, heads straight for the local magic shop. Upon entering, it is obvious that Willow is not herself: her mere presence being enough to shatter several electrical lights; on her way to the shelves containing the “black arts books“.
Willow then demonstrates a new approach to learning: when she proceeds to absorb their text (by “sucking” it out of the books and into herself). Although Willow has lost control of her emotions (running on pure grief/fury/rage), she has not yet lost her ability to help others: as she saves Buffy’s life by magically removing a bullet that would have killed her (as Warren initially shot Buffy). Despite this, Buffy refuses to help Willow destroy those responsible for Tara’s death (as the scoobie gang does not kill humans). This refusal is enough to serve as the final catalyst for Willows emotions: as she casts aside “old reliable/nerdy” Willow and becomes “Evil Willow” (with black eyes and dark veins across her face).