On the same page, we find out that Elizabeth blames herself for William's death, why? ( Support with quote)

"She told me, that that same evening William had teased her to let him wear a very valuable miniature that she possessed of your mother. This picture is gone, and was doubtless the temptation which urged the murdered to the deed."

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"Come, dearest Victor; you alone can console Elizabeth. She weeps continually, and accuses herself unjustly as the cause of his death; her words pierce my heart."