How does Macbeth behave/what is his reaction right before Duncan's murder?
In Act 1, Scene 7, Macbeth is worried about committing the murder. He worries about retribution. If he kills Duncan, someone might take revenge upon him. Macbeth says, "Bloody instructions, which being taught return to plague the inventor." (I.vii.9). He who commits the initial murder (the inventor) risks "teaching" others to murder him in return. He then considers if he has good reason ("spur") to kill Duncan. He can only conclude that his ambition must overrule any moral or logical misgivings he is having.
In this same scene, Macbeth goes back and says he will not go through with it. But after encouragement and scolding from Lady Macbeth, he commits himself to it. But he is still quite reluctant.
In Act 2, Scene 1, Macbeth imagines (or hallucinates) that he sees a dagger in his hands. He sees the blood that will result from the murder. He is trying to psyche himself into going through with it. He is using his own thoughts and words to encourage himself:
Words to the heat of deeds too cold breath gives. (II.i.69)
In other words, 'breath gives heat to deed that is too cold.' He is trying to use his own breath/words to give him the heat (courage) to go through with the deed.