I don't get the ending

She forced the doctor to admit that she was a living and thinking being. Then she turned the situation around and told her that maybe it's humans that need to be cured by the cordyceps.

As stated, she made the doctor confess that they weren't just beings that copied what they saw. They are intelligent, real creatures. Hence, "why should we have to die to save you?"

She burned the tree because she felt that her kind were the future of the world, that regular humans had no chance.