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In Justine's own words.....
“I did confess; but I confessed a lie. I confessed, that I might obtain absolution; but now that falsehood lies heavier at my heart than all my other sins. The God of heaven forgive me".
In essence, Justine wanted to go to her death absolved of all her sins. We can infer that she believed she would be unable to confess her sins before the execution if she did not confess to William's murder as well.