[Warning: The following contains spoilers from Saturday's episode of Outlander. Read at your own risk!]
The British noble has played in the moral grey area since we met him in the first season, but his final hour in the Frasers' lives not only revealed that he was willing to sell Jamie (Sam Heughan) out to the British forces, but that he was the mastermind behind Claire's (Caitriona Balfe) attack in Paris and Mary's (Rosie Day) being raped.
The Duke of Sandringham has wandered in the moral grey area since we first encountered him in Season 1. Although he claims to be a Jacobite at heart, he's never been afraid to betray Jamie or Claire to ensure his own prospects. He ruined Jamie's chances of being pardoned in the first season and was absolutely willing to trade Jamie to the British army to improve his favor with the king. Murtagh and his axe just got to him first before any real damage could be done to the Frasers or the Scottish rebellion.
Murtagh had sworn in Paris to deliver vengeance to the man who attacked them while Claire was under his watch and now he's fulfilled that oath. The decapitation was brutal, but re-enforced that crossing the Frasers has a price and that Murtagh does not make promises lightly. Jamie and Claire are lucky to have him in their corner.
The Duke's role in Mary's rape is one of the final threads left dangling from the Frasers' time in Paris. Up until now, it was thought Le Comte St. Germain (Stanley Weber) was the one behind the horrid affair, but really it was the Duke that planned the entire affair. Now that both of the Fraser foes are dead, Black Jack Randall (Tobias Menzies) is the only true villain whose fate is still undetermined.
Will justice be served to Randall before the season end? For now, the Frasers continue on with the rebellion and towards the fateful battle at Culloden Moore.
Outlander Season 2 continues Saturdays at 9/8c on Starz.