If she wanted to, then she could have easily intervened earlier and ended things right away so that she could avoid prolonging it any further in a way that could increase the risk of others coming across her.
The Tonfa were in her hands at the ready, whether it was to end its life quickly or to go to deal with another situation if something did happen.
The benefits she gained from waiting a little longer and leaving it to her Evil Spirit had proven to be effective, as both of them ended up gaining a lot.
Perhaps it might not have made that much of a difference for her Evil Spirit since it would probably be able to devour around the same amount of Life Energy, but she was able to gain much more from it after waiting a little longer.
If she could not take the Spirit back with her, then she and the Evil Spirit had to do everything there, and they had to get the most out of it within the time frame that they had available.
