Bear in mind that this a person most likely in a similar state that Miller found Julie in, the second time. I bet it's just some scared kid, I don't think it will be a big fire fight. Such an outcome would also force Holden to reconsider his belief of destroying the protomolecule.

Where as his other option was "to assist a kidnapping murderer and potentially enable them to kill again".
I would argue then that anything but allowing her to expire is far more disgusting.
If that is what made her leave then she was already gone from the start.