There were others out there in the world who were nurturing Spirits or Evil Spirits using their methods so that they could use them for some purpose.
If what they were already doing got out of control then the damage caused would be very high too.
When considering such potential risks, the methods Ash was using seemed to not seem so bad since they were helping to reduce the potential damage that could be done.
Even if she killed them and extracted their Life Energy for herself, the Spirits would still return one day once they were born again.
There was a method that could be used too which was just as helpful toward halting the whole process of their growth and their rebirth where they would begin their usual cycle of dying, taking on their form and gaining power until they met their demise again.
The method was quite simple and it involved sealing the Spirit away.
