This is not a pleasant subject, but I was just wondering about the notable descendants of Mary, Queen of Scots who met the same fate as hers. The most obvious one would be her own grandson, King Charles I of England and Scotland (executed for treason during the English civil war, pitting the so-called cavaliers against roundheads).
Others coming to mind are the Duke of Monmouth (the eldest illegitimate son of King Charles II), executed at the hands of his uncle.
You and Josť have probably named them all between you.
Like his Tudor predecessors on the English throne, King James VII/II engaged in the practice of beheading his rivals -- those relatives who dared question or contest his claim on the said throne. A nephew by blood was not going to be spared, when other monarchs in history were known to kill even their own CHILDREN for treason or rebellion.
What probably made it easier for James II/VII to kill Monmouth was that he didn't believe he was his nephew. Although Charles II had always acknowledged Monmouth as his son (albeit not as his heir) James chose to believe the rumours that he was fathered by another of his mother's lovers by whom she was already pregnant when she began her relationship with Charles. In this he followed the example of his Tudor predecessor, Mary I who also gave credence to rumours that her half-sister Elizabeth had actually been fathered by one of her mother's supposed lovers.
Later on, of course, there was the ill-fated Queen Marie Antoinette of France, consort of King Louis XVI -- both being guillotined in the middle of the so-called Reign of Terror, the height of the French revolution.
Louis XVI was a direct descendant of Mary, Queen of Scots and of Charles I but I'm not sure about Marie-Antoinette.