What is the effect of this confession on Elizabeth?

Elizabeth was devastated. Justine's guilt caused her to question her belief in human goodness.

This was a dire blow to poor Elizabeth, who had relied with firmness upon Justine's innocence. "Alas!" said she. "How shall I ever again believe in human goodness? Justine, whom I loved and esteemed as my sister, how could she put on those smiles of innocence only to betray? Her mild eyes seemed incapable of any severity or guile, and yet she has committed a murder."