For me, Duncan’s own servants killed the king under instruction of his own children. The reason why it is unbelievable that his children can kill him is that they would have inherited it anyway. But it is really obvious that the main reason is that for the ambition for power .And it also says something about not only Macbeth’s, and his wife's cunning that they foresaw this reaction but also about Ross and Macduff that they just believe everything.