I am not saying Yosetsu's actions were justified or ok. I am explaining her viewpoint and the fact that while it doesn't justify what she did or what she became it does have some basis.
Her actions aren't based around simple revenge. We see many people in the occupied territories turn a blind eye to cruelty and injustice for the sake of survival. In her eyes she is just turning that reality back on the people who chose safety when she was the victim. As I said, in her world view you are either the abused or the abuser and she was put on top. Just look at her choices. She forces that fisherman to kill others so he might live. She is making them do anything to survive, even if it means killing their own families.