She regarded her as a personal and a violent enemy.

She knew that schemes for assassinating heretics were very familiar in that age, and generally approved of by the court of Rome and the zealous Catholics.

Her own liberty and sovereignty were connected with the success of this enterprise; and it cannot appear strange, that where men of so much merit as Babington could be engaged by bigotry alone in so criminal an enterprise, Mary, who was actuated by the same motive, joined to so many others, should have given her consent to a scheme projected by her friends.

We may be previously certain, that if such a scheme was ever communicated to her, with any probability of success, she would assent to it; and it served the purpose of Walsingham and the English ministry to facilitate the communication of these schemes, as soon as they had gotten an expedient for intercepting her answer, and detecting the conspiracy.

Now, Walsingham's knowledge of the matter is a supposition necessary to account for the letter delivered to Babington.
As to the not punishing of Nau and Curle by Elizabeth, it never is the practice to punish lesser criminals, who had given evidence against the principal.
But what ought to induce us to reject these three suppositions is, that they must all of them be considered as bare possibilities.