I read something on one of these threads, something he wrote on his blog about longing for companionship. My thought has been that he kept the girl as a companion. Being female, he was not sexually attracted to her. She was an obedient companion to keep him from being lonely. He has huge mother issues too.
I have wondered the same thing after reading about his lonliness...I certainly don't think he kept her alive because of an attack of conscience.