Macduff discovers Duncan's body in Act II, Scene III. Initially, he is shocked that somebody would commit the murder of a king. For Macduff, the king is anointed and appointed by God: note, for example, how he uses the phrase "God's temple" to describe the king's bedchamber. Consequently, Macduff is outraged by this murder because it is offensive to God.
Macduff is also very upset by the discovery of the body. He tells Macbeth and Lennox that he does not want to talk about the horrible sight he has just witnessed:
Do not bid me speak.
He also refuses to tell Lady Macbeth because he thinks the news is not fit for her ears:
O gentle lady,
'Tis not for you to hear what I can speak.
Finally, for Macduff, the discovery of the body is akin to the end of the world. He calls it "the great doom's image" while he rouses the royal household from their beds. For Macduff, this is a clear act of treason, which he takes extremely seriously.
Macduff is initially annoyed that he's been knocking on the door and Macbeth has taken so long to answer. Macduff is supposed to wake up Duncan, but he has had to wait for Macbeth. Macduff takes his duties to the king seriously because he is loyal.
Upon discovering Duncan's body, Macduff is horrified. At first, he can not even find the words to describe what he has seen. He says "Tongue nor heart / Cannot conceive nor name thee." (II.iii.67) He then finds the words. He says "Confusion hath made his masterpiece." (68) He means that it is a perfect work (masterpiece) of destruction (confusion). He calls the murder sacrilegious because it seems so unholy and because kings were thought to be ordained by God. He tells Macbeth and Lennox that the sight is so horrible, it will turn them to stone. He references the Gorgon (Medusa), a monster so horrible to look at that she would turn one to stone.
Macduff cries out that it is treason and summons Malcolm and Donalbain from their sleep. Macduff is clearly outraged and disgusted. As a loyal subject to the king, he is extremely disheartened. And the sight of Ducnan's murdered boy must be gruesome, given Macduff's horrified reaction.