Something's Rotten(1979)

The queen of an unnamed European country is being pressured by her Prime Minister to abdicate the throne in favour of one of her sons. But she wrestles with the question which son, the older but emotionally unstable Prince Calvin or the younger but more mature Prince George, will be named as her successor. Meanwhile, a series of murders of the palace staff is being committed by an unidentified “Skulker”, whom the queen begins to suspect may in fact be one of the princes.