Well, this woman obviously had values. She had values of non-violence, values and principles that valued human life, and she wanted to save people from oppression. So by her following her principles, she justified her existence. If she had gone against her principles and had not tried to save those she felt were weak and who could not save themselves, she might have felt within herself that she couldn't justify being on this Earth, because she didn't follow her heart and what she truly believed in.
It depends on what types of principles you have in your heart, whether you go with them, whether you follow them, and whether you feel good about yourself and your own existence on this Earth - this is our duty. I once heard a Thai teacher say, "Tom na-tee", which means "do your duty" and he was using it in the Buddhist terms of following the right principles, following the Noble Eightfold Path in whatever situation you are in.
So, in order to justify her existence in her terms, Irena Sendler had an idea of what human life is worth, and what following those principle meant to her. And that she didn't do it for fame, she didn't do it for gain, she didn't do it for praise, and she didn't do it for pleasure. She did something extremely difficult because she followed the compassionate principle in her heart - that was her duty - to save these human beings; she saw them as a human being and not as Jewish child whose life was not worthy of existence.