It's after Grimmjow's first attack that Rukia makes the decision. She thought that with her powers back she would be able to fight once more, but she can do nothing against Aizen's Espada. She is sick of being helpless and there is only one way to power she knows. She is not like Ichigo, who can achieve Bankai after a couple days training with Yoruichi-sama. She's not even sure Bankai is possible for her, and if it is at all, it will be decades before she reaches that level of power.
From what Ichigo doesn't say about his zanpakutou, Rukia can tell that they seem to get along pretty well. The same cannot be said for her Sode no Shirayuki. Sode no Shirayuki has never liked her and infuriatingly enough, never showed her even the tiniest bit of respect until she killed Kaien-dono.
She is cruel, arrogant, and demeaning, and every single Dance Rukia has managed to eke out of her came with strong resistance and a healthy dose of contemptuous and degrading remarks. Sode no Shirayuki may be one of the most beautiful zanpakutou in the Soul Society, but despite this (or perhaps because of it,) she has never been satisfied with her spectacularly average wielder. There is a reason Rukia has always preferred Kidou. Kidou has never mocked her failings or laughed at the tragedies of her life.
Bankai is unobtainable, further development of her sword's powers would take too long, and she has already mastered up to the 80s in Kidou. She will not be helpless. She cannot stand to be a burden on Ichigo or anyone else any longer. She needs power and there is only one fast way to obtain it. There is yet again only one path before her.
Urahara does not approve.
"I don't think you realize what you're asking, Kuchiki-san," he says, looking shocked as she kneels across from him in the backroom of his shop. "This is not something anyone has done voluntarily. The risks are astronomical."
"This is not a decision I have come to lightly, Urahara-sama," Rukia says firmly, fists pressed to the floor in her resolve. "But I find there to be no other alternative."
"I'm afraid I can't help you, Kuchiki-san," Urahara refuses, holding out his hands in front of him imploringly. "It was always a risk in Kurosaki-san's case, but to purposefu-"
"You. Owe. Me." Rukia says furiously.
It's the first time she's acknowledged his betrayal of her and it has the desired effect and stops him right in his tracks. Rukia didn't want to resort to this, honestly thought that he'd welcome the opportunity to acquire yet another test subject, but it appears that he's guiltier about his attempt to destroy the Hougyoku than she thought and she'll take advantage of that if she must.
Urahara seems shocked into speechlessness, something she's never seen happen to him in all twenty years that she's known him. For a long moment, he considers her, something like shame bleeding into his usually enigmatic expression.
"Kuchiki-san," he says softly, as close as she's ever heard him to pleading before. "You'll never be able to go back. Even if the war ends tomorrow and we are all pardoned by the Soul Society, they'll never take you back. I don't think you want that. You loved being a shinigami."
Rukia can't feel even an ounce of pity for him. Even as he pleads for her soul, all she can hear is her own voice, begging for Ichigo's life in the summer rain.
"I did," Rukia agrees coldly. "I really did. But that is irrelevant now. I need power to fight in this war and you are going to give it to me. I told you, did I not, all those years ago, that I would give my life to be of use to you. Why not my soul?"
"Have you talked to Kurosaki-san about this?" he tries one last time, even though he must know Ichigo is in the midst of intensive training with the Vizards. "No...that's a stupid question, isn't it."
Rukia says nothing, not breaking eye contact with him for a second, fists pressed so hard to the tatami that there will surely be indentations in her knuckles when she finally rises.
"Alright," he acquiesces softly, turning his head away in shame. "You won't regret this?"
"I will not," Rukia says harshly, standing and following him further into the shop as he calls for Tsukabishi-san. "And neither will you."
"That's where you're wrong, Kuchiki-san," Urahara says sadly, adjusting his striped hat to fall further over his eyes. "I already do."
Ichigo goes nearly insane with rage when he's found out what she's done, and even the Vizards are disturbed by her choice to allow a Hollow into her heart. The Soul Society might have forgiven her desertion in the light of Aizen's incredible power, but they will never forgive her this.
It's...a relief, in a way, to finally get closure from that part of her life. She really can never go back now, and now there's nothing else to do but to look towards the future.
A/N: Flashback time! Any questions or comments, don't hesitate to review!