So her family abandoned her or even bullied her back then. Perhaps the reason the prince looked for her back then was not to kill her, but to save her?? 
Comment by Humpback on A Capable Maid - Chapter 5
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@Humpback she was illegitimate child born from commoner (maid i think) and king. King hated that so he didnt helped her mother until she died. He let her stay in palace but in abadomed wing so she could grow/die in isolation. Prince killed whole royal family and wanted to kill her to use it as achievment to bevome crown prince but he couldnt find her so emperor still allowed him to become one with quest to destroy last royal blood