Mary, Queen of Scots
Escaping political unrest in her own country, Mary, Queen of Scots came to England in 1568, hoping her cousin Queen Elizabeth would help her. On the one hand, Mary was logical in this thinking because Elizabeth was not only another Queen ruling alone, but also her kin as the two were cousins. However, on the other hand, Elizabeth saw her presence as a threat because Mary could become a figure to her Catholic opponents. So Elizabeth imprisoned her cousin and gave her no hint on whether she would ever be released.
It is unclear if Mary herself started and led the Catholic plots to overthrow Elizabeth or if she was simply persuaded into a plot that was already in action by another leader, Anthony Babington for example. Mary was eventually found guilty of conspiring to assassinate the queen in the Babington plot; her own signature on secret letters securing her own death.
Elizabeth had no choice but to execute Mary, as was the law. Evidence shows that Elizabeth did not like having to behead her cousin not out of kinship or love most likely, but out of fear of the repercussions of killing another queen.