At the beginning of Act II in Shakespeare's Macbeth, Macbeth has just been talked into going ahead with the assassination (Act I:vii). He is awaiting the signal from his wife that the grooms are asleep/drugged and he can go ahead with the killing. He meets Banquo and Fleance on the way and makes small talk. Banquo raises the topic of the weird sisters' predictions, and Macbeth lies and says: "I think not of them." Macbeth's first concern in these two scenes is to not get caught. He of course doesn't want Banquo to know that he has pretty much not thought of anything except the predictions since he first heard them.
When Banquo and Fleance exit, Macbeth suffers the vision of the bloody dagger. His sense of guilt is overwhelming. His brain is "heat-oppressed."
As he moves toward Duncan's chamber, he wishes to be hidden, to avoid the guilt and again to avoid being seen and being caught:
...Thou sure and firm-set earth,
Hear not my steps, which way they walk, for fear
Thy very stones prate of my whereabout,
And take the present horror from the time,
Which now suits with it.
When Macbeth returns to the stage in Scene 2 he has killed Duncan. His guilt continues to thwart him, causing him to be unable to join in with a prayer he overhears by saying Amen, and causing him to hear voices. He worries that he will never be able to wash Duncan's blood off of his hands, and as he exits the stage he cries:
Wake Duncan with thy knocking. I would thou couldst!
Describing Macbeth's feelings during these two scenes as concerns is probably an understatement. He is terrified and seems to be losing control.
At the beginning of Act II of Macbeth, Macbeth's fears begin to take over him. He has gone through with the murder of King Duncan, and he fears that someone in the castle knows about his crime. In the beginning of the second act, he hallucinates the dagger which is the symbol of his fear and inner conflict. Macbeth does not know how to deal with his concerns, so he becomes a bit irrational while trying to cope with his guilt. Moreover, he knows that his ascension to the throne has been got by ill means and that the witches have foreseen that he will have no heir to the throne, so he tries to think of a way to secure his position.