Rabastan knew it would be almost a chore to take Alecto with him, but it was definitely better than leaving her alone with Bellatrix. He was charitable sometimes... especially knowing that if Alecto died, it would be partially his fault for not attempting to keep her away from Bellatrix when she was at her most dangerous. He was the one who always rationalized things, and that seemed to be how it would stay.
"You'll get the hang of it soon enough," he told her. "After this time, you won't need to come with me again."