One of the short term causes that lead up to Mary’s execution was the St. Bartholomew’s day Massacre, 1572. This was when thousands of Protestants were massacred by the French Catholics. This suddenly increased Elizabeth’s fear and suspicion of English Catholics and increased her hostility towards Mary. Moreover, when Catholic Priests arrived in England in the mid 1570’s, Elizabeth’s fear of the Catholic faith spreading was confirmed. The priests held secret religious services for English Catholics. In these services, priests tried to convert people to support Mary and not Elizabeth. This contributed to the worry of Catholics rebelling against Elizabeth.
The Throckmorton Plot in 1583 also contributed towards Mary’s execution. Francis Throckmorton was a Catholic. The plan involved an invasion of England led by the Duke of Guise and paid for by Philip of Spain and the Pope. The aim was to overthrow Elizabeth and put Mary Queen of Scots on the throne. At first, Elizabeth was led to believe that Mary was not involved, but when Throckmorton was put under torture, he confessed to Mary being involved.
In 1584, William of Orange was murdered by an unnamed catholic. This sparked concerns that Elizabeth was the next protestant leader to be killed. Because of this, the Privy Council decided sign a document called the Bond of Association. In the event of Elizabeth's murder, those who signed the Bond promised to punish not only those responsible, but also anyone who might benefit from Elizabeth's death - clearly a reference to Mary Queen of Scots.
England’s war with Spain in 1585, was also a factor that could have contributed to Mary’s death. Spain was angry with Elizabeth’s treatment of Mary and Phillip of Spain sent out ships to invade England.. He was told that if he did this, English Catholics would rebel against Elizabeth. This gave Mary hope.
The trigger cause of Mary’s execution, was the Babington Plot. The details of the plot were by now familiar - a Catholic rising, supported by a foreign invasion to secure Mary's release and Elizabeth's assassination. Babington, like Throckmorton before him, was used to carry messages to and from Mary and may have been the intended assassin. The difference was that Walsingham ( Elizabeth’s hired spy ) learned early enough about this plot to be able to intercept letters exchanged between Babington and Mary (which were hidden in beer barrels) without either of them suspecting anything. In June, Babington sent a letter to Mary telling her about the plot. Mary's reply, on 18th July, contained her approval of the plot, including plans for the murder of Elizabeth. This finally gave Walsingham the vital proof he needed of Mary's guilt. Mary's trial began on 15th October. She was found guilty, however, she was not executed until the 8th February 1587.
In conclusion, all 8 of these factors contribute to Mary’s death. They all fit together, like a jigsaw, to create the final picture. Without one piece, the puzzle would not be complete. For example, if the Babington Plot had not have happened, Mary might have survived, and lived on to be the Queen of England. So, in all , you can not determine one specific cause that lead to her execution ; it’s impossible. But, it is possible to see that all of them happened for a reason, and maybe it happened for the best.