Macbeth was probably always a sociopath. It just took the witches to spur him on. They would not have succeeded if he had not already had it in him. Remember that in Act 1 he is described as cutting someone in two. There are less hideous ways to win a war.
First, Macbeth hallucinates. If you assume the witches are real (if Banquo saw them too, and Banquo was not a part of the hallucination), the dagger certainly was not. In Act 4, Scene 1, it seems to be only Macbeth that sees the witches. The dagger is a vivid hallucination, but it seems to give Macbeth the courage to continue to murder.
Is this a dagger which I see before me,
The handle toward my hand? Come, let me clutch thee.
I have thee not, and yet I see thee still.
Art thou not, fatal vision, sensible
To feeling as to sight? Or art thou but(45)
A dagger of the mind, a false creation,
Proceeding from the heat-oppressed brain? (Act 2, Scene 1)
Macbeth is clearly disturbed after murdering Duncan. He asks his wife is she has heard a noise, and then says that he heard a voice.
There's one did laugh in's sleep, and one cried,(30)
That they did wake each other: I stood and heard them:
But they did say their prayers and address'd them
Again to sleep. (Act 2, Scene 2)
Notice that he does not feel bad about what he did. He just worries about being caught. He doesn’t show true remorse.
Macbeth worries that he could not say “amen” when he walked by the voice that said he murdered sleep.
One cried, “God bless us!” and “Amen” the other,
As they had seen me with these hangman's hands.
Listening their fear, I could not say “Amen,”
When they did say “God bless us!” (Act 2, Scene 2)
You can interpret this either as him worrying about divine punishment, or wondering if he has a conscience. The thought of having a conscience seems to bother him!
Will all great Neptune's ocean wash this blood
Clean from my hand? No, this my hand will rather
The multitudinous seas incarnadine,
Making the green one red. (Act 2, Scene 2)
Macbeth’s psych is starting to shatter. This preoccupation with blood on his hands does not prevent him from killing others. If he were not a sociopath, he would have cared and possibly stopped killing.