Elizabeth, in chapter 7 of Frankenstein, blames herself for the murder of William because she feels that she provided the murderer with the motive to kill the boy: Earlier that day William had been bugging her to let him wear a little locket with a tiny picture of his grandmother in it. The locket was valuable, and because of the mark on the boy's neck (and the fact that the locket was missing) Elizabeth believes that the murderer killed him in order to steal it. Thus, by allowing the boy to wear the valuable locket in the first place, Elizabeth feels she contributed to the death of the child.
"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 murderer to the deed."
"She weeps continually, and accuses herself unjustly as the cause of his death..."