What did William have on him that brought the murder back on Justine?

William had a likeness of his mother, a miniature, which was found in Justine's pocket.

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. We have no trace of him at present, although our exertions to discover him are unremitted; but they will not restore my beloved William!

During this interval, one of the servants, happening to examine the apparel she had worn on the night of the murder, had discovered in her pocket the picture of my mother, which had been judged to be the temptation of the murderer.