I would say the most important moment you need to focus on is in Act II scene 3, when Macbeth emerges from Duncan's bedchamber, and, supposedly, in a fit of rage and grief at seeing Duncan's blood on his two grooms, kills them both before they have had a chance to testify. Note in particular Macduff's very significant line, where he asks Macbeth why it is that he did this:
Wherefore did you so?
What is important to realise is that Macbeth has called down massive suspicion on himself because of the way in which he killed them so quickly and swiftly without letting them be questioned first. This can be noted as the first moment when Macduff begins to suspect Macbeth and his involvement in this crime of regicide. To support this view, you might like to look at Macbeth's response to Macduff's question, and in particular how Lady Macbeth feels her husband is doing such a bad job of clearing himself of suspicion that she feels the need to faint so as to distract everybody from her husband's pathetic answer.
and the reason why please! thanks