Who suspects Macbeth of the murders of Duncan and the guards?
Macduff suspects Macbeth of murdering Duncan and the guards. Macbeth very reluctantly accompanied Macduff and Lennox when they came to wake the King early in the morning. Macbeth was behaving in a weird manner because of his guilt, remorse, and dread of the uproar that was about to occur when Macduff discovered the body. Macduff did not understand Macbeth's cold, stiff manner and curt replies at the time. He thought Macbeth was angry at being dragged out of bed by his persistent knocking. But later when he reflected on it, Macduff realized the truth. That was why he refused to attend Macbeth's coronation banquet.
Banquo also suspects that Macbeth murdered Duncan and the guards. Banquo is the only person outside of Macbeth and his wife who knows that the Three Witches had promised Macbeth he would become king of Scotland. When Act III, Scene 1 opens, Banquo is reflecting on the matter in a soliloquy beginning as follows:
Thou hast it now: King, Cawdor, Glamis, all,
As the weird women promised, and I fear
Thou play'dst most foully for't:
There may be many other thanes who suspect Macbeth on the principle of cui bono, i.e., who benefitted from Duncan's death? But the others are wise enough to keep their suspicions to themselves. Lennox was with Macbeth when Macduff discovered Duncan's body and raised a great outcry. But Lennox was young and guileless at the time and suspected nothing. Some years later, however, Lennox has become much more sophisticated and voices his suspicions of Macbeth in carefully guarded irony.
Men must not walk too late.
Who cannot want the thought, how monstrous
It was for Malcolm and for Donalbain
To kill their gracious father? Damned fact!
How it did grieve Macbeth! Did he not straight,
In pious rage, the two delinquents tear,
That were the slaves of drink and thralls of sleep?
Was not that nobly done? Ay, and wisely too,
For ’twould have anger'd any heart alive
To hear the men deny't. (Act III, Scene 6)