"First, as I am his kinsman and his subject, / Strong both against the deed" (I.7.13-14).
"O, full of scorpions is my mind, dear wife! / Thou know'st that Banquo, and his Fleance, lives" (III.2.41-42).
"From this moment / The very firstlings of my heart shall be / The firstlings of my hand ... The castle of Macduff I will surprise, / Seize upn Fife, give to th' edge o' th' sword / His wife, his babies, and all unfortunate souls / That trace him in his line" (IV.1.163-170).
At the beginning of the play, Macbeth has not yet been corrupted by power or the ambition to gain more power. This is proven by the fact that he has many reasons for not wanting to follow through with Duncan's murder. For instance, here, he is thinking that he should not murder Duncan because he is Duncan's subject and is part of his family.
After murdering Duncan, Macbeth is still not fully evil and corrupted until this shift in his character. This shift occurs when he decides that Banquo and Fleance pose a danger to him and the throne and decides to kill them. When one decides to kill their own friend and his son to maintain power, they have become evil and corrupted.
After Macbeth's shift, he has been becoming eviler and more corrupt up to this point in the play. He does not care who he murders at this point, as he feels everyone is a danger to him and the throne. This is elucidated by the fact that he not only wants to kill Macduff, but seize his entire town as well.