The general opinion is that his sons Malcolm and Donalbain had hired his grooms to kill their father. This was supposedly proven because they took off after the murder was discovered, a sign of guilt. This is explained by Macduff in Act II Scene 4.
Banquo. "Thou hast it all--king, Cawdor, Glamis, all as the weird women promised, and I fear thou playedst most foully for it." Act III Scene 1
Ross, maybe. But definately Banquo, as we know from the speech he gives at the beginning of Act III.
A man called Banquo.
Macbeth
Macduff
Macbeth..
Lennox
Both Lennox and the lord he is speaking with suspect Macbeth of killing Duncan and Banquo
Macbeth believes he has to kill Duncan's guards in case they heard anything happen in the room while Macbeth was killing Duncan.
Lady macbeth wants macbeth to kill king duncan because she wants to be queen. King duncan has a lot of trust with macbeth so no one will suspect it. And she also persuades him by saying he can be king, and have all this power.
And then Macbeth asked Macbeth if he confronted Macbeth on killing Macbeth with Macbeth.
The king
Both Lennox and the lord he is speaking with suspect Macbeth of killing Duncan and Banquo
Macbeth believes he has to kill Duncan's guards in case they heard anything happen in the room while Macbeth was killing Duncan.
Lady macbeth wants macbeth to kill king duncan because she wants to be queen. King duncan has a lot of trust with macbeth so no one will suspect it. And she also persuades him by saying he can be king, and have all this power.
Lady Macbeth talked Macbeth into killing his cousin, King Duncan of Scotland, who was a guest in his house at the time.
And then Macbeth asked Macbeth if he confronted Macbeth on killing Macbeth with Macbeth.
The king
The reaction between Lady Macbeth and Macbeth is initially very different after they kill Duncan. Lady Macbeth is enthusiastic but Macbeth regrets his actions.
Killing him in his sleep. Scandalous!
She talks her husband into killing Duncan.
"Sleep no more, Macbeth doth murder sleep."
Macbeth was not a vassal of King Duncan. Macbeth's act of killing King Duncan would be considered regicide because he unlawfully killed his king, who was his sovereign and ruler.
After everyone found out Duncan was dead, Macbeth claimed that it was the guards who had slain King Duncan, as he made sure to smear the blood on their drunk, passed-out bodies.