Mary's second husband Lord Henry Darnley was a vindictive, alcoholic, spiteful, womanising, pig-f*cker he had all the necessary attributes to become Prime Minister but it also meant he had a lot of enemies. There were plenty in the Scottish nobility with motive to want Darnley dead, his murder in the Scottish Gunpowder Plot in February 1567 is Scottish history's biggest 'who dunnit?' 

*note to English listeners, when we do a gunpowder plot here in Scotland we make sure the guy actually dies