Whom does Macduff suspect of the murder?

Macduff has good reason to suspect Macbeth of murdering King Duncan. Macbeth was with him when he discovered Duncan's body. Macbeth had been acting very strangely ever since encountering Macduff when the Porter finally opened the gate and let him in. At the time Macduff did not understand why Macbeth seemed to stiff and cold. He thought it was merely because his pounding at the gate had awakened Macbeth from a sound sleep and made him angry. But Macbeth had never even been to bed. He had wanted to pretend to have been in bed asleep when the murder had been committed, but the unrelenting pounding at the gate had finally forced him to come down and find out what was going on. He could hardly pretend to be asleep with all that racket. Shakespeare invented the knocking at the gate because he wanted Macbeth to be present when the body was discovered.

Then there is the fact that Macbeth murdered Duncan's two grooms. He must have felt that he had to do it in order to prevent them from protesting their innocence. He also fears that one of them might have seen him in the king's bedchamber. Since there are two grooms, each of them could attest to the other's story. If the grooms had survived and the thanes had questioned them, the grooms would never have said that they were paid to commit the murder. Even under torture, which was sure to be used, they could not invent such a tale. The conclusion would have to be that someone had slipped into the chamber and killed Duncan while the grooms were asleep. Since Macbeth had murdered the grooms, it looks to Macduff as if he did it to silence them and protect himself. Macbeth knows that killing them looks suspicious. He says:

O, yet I do repent me of my fury
That I did kill them.

And Macduff immediately asks:'

Wherefore did you so?

Macbeth could only repent his act because it prevented everyone from questioning the grooms. Lady Macbeth sees that her husband is not doing a good job of lying. She pretends to have a fainting fit in order to create a distraction and cries:

Help me hence, ho!

Macduff is already suspicious of Macbeth. When Macbeth manages to get himself elected king, that is convincing proof that Macbeth was the perpetrator. It is a question of cui bono? (Who benefits?) The man who becomes king is probably the man who committed the murder. When Macbeth becomes king, everybody will begin to suspect him of being Duncan's murderer. Cui bono?

Macduff is more suspicious of Macbeth than any of the others because he was with Macbeth and observed his guilty behavior. He hates Macbeth for killing the king, and Macbeth hates and fears Macduff because he knows Macduff knows he is the guilty party. That is one of the reasons why he takes such a draconian reprisal against Macduff's entire family when the Thane of Fife flees to England. By wiping out Macduff's family and his entire household, Macbeth hopes to deprive Macduff of any reason for coming back. Macbeth knows he is a villain. He can't look Macduff in the eye because Macduff knows what he is. When they meet on the battlefield, Macbeth says:

Of all men else I have avoided thee.

And then he learns to his horror that Macduff "was from his mother's womb untimely ripped." No doubt the poor mother died as a result of that primitive operation, and now Macduff is motherless as well as having had his wife and all his children exterminated. Macduff is like a monster. He is Macbeth's worst nightmare.