I don't want to drag things out too long. The lamia is a definite threat if let loose, and there's no guarantee what her alignment would be if you destroy the core.
If you tie her up, she'll either get killed by something else, or get free. If you let her go, she might hurt others or come after you. If you bring her with you, she has many subtle powers to screw with you, and dungeons are dangerous enough.