Macbeth’s mental state can be seen as starting to deteriorate when he talks to dagger as if it is a person (use of persoification.) He is talking to inanimate object that isn’t even there. “Come, let me clutch thee.”

Macbeth also starts to question his mental state himself in the solioquoy 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
A dagger of the mind, a false creation,
Proceeding from the heat-oppressed brain?’

Macbeth ends uo having a conversation with himself which also another reason believe mental state is starting to change as he talking to himself about whether the dagger in front of him is real or just a figment of his imagination.