I think James Stewart, Earl of Moray, later Regent of Scotland, was a bad bastard, but that judgement, like everything else in history, is open to dispute.
James was the illegitimate son of James V of Scotland and his favourite mistress (whom he very nearly married), Lady Margaret Erskine.
No time now to explain, but hopefully back later. Was James an utter bastard in his dealings with Mary, Queen of Scots, or was he simply an astute and able politician driven to desperate measures by the idiocy of his young half-sister?