In the end, she despised it, but she wanted to lessen the pain and block out their hearts from beating. Her demon mocked her on 'how good it would be' to sate that hunger.
She walked for 2 years searching for a home after the Hell Mouth collapsed because of Spike. She didn't even know he was alive, until she got a shocking view of a TV screen, showing a broadcast previewing Angel and Spike's new Wolfram and Hart.
It was a celebrity moment. A drive to achieve something 'BIG.'
Spike and Angel were just dealing with rebuilding their lives after the final battle and loss of their friends like Gunn and Illyria. Not to mention Wesley and Lorne's decision to leave, having been too tired to fight much longer. Too forlorn by deaths.
Buffy hid out. She didn't want to tell them she left because she was turned and by some 'lowly minion.'
It would shock Spike and anger Angel, successfully ruining any hunger lust she had planned. Well, if the Scoobies didn't want her, she could always kill...?
She has a soul. She wants to kill. Wants destruction. So, what does that make her...? A Vampire Freak...?
Time for Plan B.