Macbeth himself gives several reasons why the murder of Duncan is such a terrible deed in his soliloquy in the final scene of Act I. Vacillating over whether to kill Duncan, he observes that the deed is doubly foul because Duncan is "here [meaning Macbeth's castle] in double trust." First, as we learn earlier in the play, Macbeth is Duncan's cousin, making him both the king's "kinsman and his subject." So not only is he killing a king, but he is killing his own flesh and blood. He is also his host, which Macbeth says obliges him to "against his murderer shut the door." Macbeth's honor partially depends on his ability to show Duncan hospitality, making the murder even more odious. Duncan also has been, according to Macbeth, a good king:
Besides, this Duncan
Hath borne his faculties so meek, hath been
So clear in his great office, that his virtues
Will plead like angels trumpet-tongued against
The deep damnation of his taking-off...
Macbeth lives in a violent society, one which certainly encourages the killing of those who have wronged him, but he has to admit that Duncan has done no such thing. Rather, he admits to himself, he has "no spur/To prick the sides of my intent, but only/ Vaulting ambition." It is his ambition and nothing else that drives him to commit the murder. There is no justice in it. So as Duncan's subject, host, and kinsman, Macbeth can find no legitimate reason to commit such a horrible deed. This is why the murder is so terrible.