why does Chaya feel guilty about Rueven being taken by Commandant Breuer? What made her believe that "we are all monster"?

Chaya yells for Rueven to run, as he's walking with a bloodied knee; her warning comes just as the Commandant stops his car to question the boy...... then she tells him Rueven has no family, She believes it to be her fault because she should have said he was her brother.

Sorry, she names everyone monsters including herself because they are to frightened and even selfish to save another if it means they may possibly be taken themselves.