Even before Macbeth suspects anyone is plotting against him, he feels guilty, paranoid, and always anxious that his crimes will be discovered and/or he will be overthrown. In Act 3, Scene 1, Macbeth determines to kill Banquo because the witches had prophesied that Banquo's descendants would be kings:
There is none but he
Whose being I do fear, and under him
My genius is rebuked as, it is said,
Mark Antony's was by Caesar. He chid the sisters
When first they put the name of king upon me,
And bade them speak to him. Then, prophet-like,
They hailed him farther to a line of kings. (III.i.55-61)
Banquo himself never makes a threatening gesture, but Macbeth fears his descendants. So, to make sure his own sons will reign after him, Macbeth determines to kill Banquo and Fleance. In a sense, it is here that Macbeth feels that the fates are at least partially against him so he must eliminate any potential enemies before such fates could materialize. Macbeth's anxiety is amplified when he sees Banquo's ghost at the dinner table. He has clearly become paranoid, haunted by the victims of his crimes and by the possibility that others might rise up against him.
As Macbeth becomes more anxious and paranoid, his determination to protect his power also increases. In Act 5, Scene 3, even when some of his supposed loyal followers desert him, he still insists on fighting and is actually quite confident that he will prevail because of how he misinterprets the witches' predictions about the future. Thus, he has become delusional as well.
All mortal consequences have pronounce me thus:
'Fear not, Macbeth. No man that's born of woman
Shall e'er have power upon thee.' Then fly false thanes,
And mingle with the English epicures. (V.iii.5-8)
Paradoxically, as his paranoia and anxiety increase, so does his confidence and determination.