Elizabeth had always believed that executing Mary would lead to bigger problems, such as an international backlash, which is why she held off executing her for so long. ... Elizabeth wrote to King James apologising for the death of his mother, Mary.

Do you think Elizabeth was right to execute Mary?

The most significant threat that Mary posed to Elizabeth, was the threat of her accession. She had an unimpeachable claim to the throne; her marriage to Darnley made this even stronger. ... To conclude, Elizabeth was justified and correct to execute Mary Stuart on the grounds of the threat she posed to the Queen.

Why did Elizabeth rule instead of Mary?

Her older half-sister, Mary, had lost her position as a legitimate heir when Henry annulled his marriage to Mary's mother, Catherine of Aragon, to marry Anne, with the intent to sire a male heir and ensure the Tudor succession. ... Elizabeth was declared illegitimate and deprived of her place in the royal succession.