Theory: The Source of the Addiction
WARNING: Spoilers for S2 - S7 of Buffy and S4 of Angel.
Willow’s original reasons to learn magic were purely to help Buffy - it wasn’t that she wanted to feel powerful or superior, or for personal gain.
Giles was adamant that The Ritual of Restoration was far too dangerous for Willow to perform, as well as well beyond her skill level; he wagered even he could not complete it. He warned her that it could unleash something within her that there would be no turning back from.
In “Becoming”, she began the ritual in English to little effect. Her voice was weak and lacking confidence. But then something took over and she began speaking the original Kalderash version, seemingly something other than herself channeling through her.
I think this was the moment something awakened within Willow that led to her addiction. Up until then, only Kalderash women had performed the ritual, but Willow does not carry that ancestry. An outsider using that ritual must come with some type of cost, otherwise it would not be reserved for only a certain population of people.
My theory is the cost of restoring Angel’s soul was the corruption of Willow’s.
The effects weren’t immediate, but gradual, the first big example being in season 4 with her spell for her will to be done. This was the first violation of Willow’s own code for magic to be done for good and not self gain. Her soul is still mostly pure, however, and she is able to reign herself in, but things only snowballed from there in season 5, as Tara expressed her fear of Willow’s rise in proficiency.
I think what ultimately sealed her fate, or at least accelerated it, was the restoration of Buffy. Willow restored not only the soul, but her entire being—physical and mental, too. I imagine the small dark spot on her soul expanding. The manifestation of the evil demon/entity in episode 3 wasn’t the only price of the spell.
Her addiction and selfishness ramps up rather quickly; she is impressed and cocky with herself, which is out of character. Her spat with Giles only furthered this. Then Amy came in the picture, and Willow was primed for addiction. By Tabula Rosa she is a complete addict, only snapping out of it after hurting Dawn and driving Tara away. But the stain on her soul is too much, and Tara’s death sends her over the edge.
After her relapse, Giles brings Willow to the Coven in England who reconnect her to earth magic, and that I believe helped begin her soul’s healing process. She taps into this to also heal physically after being attacked by Gnarl.
** Angel Spoiler **
I think proof of her healing is when she goes to LA and performs the Ritual of Restoration a second time.
This time around, she doesn’t need to channel anything - Willow is the power source. Her journey through the corruption and healing of her soul made Willow worthy of performing the Ritual without side effect, just as the Kalderash women were seemingly able to do.
Anyway, can you tell I had some THC today? I’d love to hear thoughts on this - agreeing or disagreeing!