There is enough evidence that Mary Queen of Scots conspired to have her husband Lord Darnley killed. But why was that considered a criminal act?
Henry the eighth had a couple of his wives beheaded and that was considered fine. Yes that was because they cheated on him.
But so what? didn't kings and queens in those days have divine right to do pretty much what they wanted? Why did people even at that time make such a big fuss about the fact that Lord Darnley had obviously been killed? Is it because Mary was a woman that different standards applied to her?
Henry the VIII had Anne Boleyn and Katherine Howard tried and sentenced to death. Whatever you feel it was due process or not, the process itself makes it fall under the category of « dispensing Royal justice ».
Mary Queen of Scots either conspired or assented to bunch of her court nobles assassinating her own husband in the middle of the night without even the process of a death warrant. That falls under committing murder and parricide - had she not been Queen of Scots, it would have even been petty treason.