When Macbeth first hears the prophecy that he will be king, he "start[s] and seem[s] to fear" the Weird Sisters' words, according to Banquo (1.3.43). He does not really know what to think, at first. We later learn from his wife's speech that he is "full o' th' milk of human kindness," and so it seems unlikely that he would jump right to the idea of murdering his friend, cousin, and king (1.5.17). However, when Duncan names his son, Malcolm, as heir to the throne, Macbeth's character begins to change. He says,
Stars, hide your fires;
Let not light see my black and deep desires.
The eye wink at the hand, yet let that be
Which the eye fears, when it is done, to see (1.4.57–60).
Now, he asks for darkness so that no one will be able to see his dark dreams of becoming king. He does not want his eye to see what his hand is doing, because he knows what his hand will do is wrong. He is clearly capable of contemplating murder now.
Macbeth, however, soon tells his wife, "We will proceed no further in this business" (1.7.34). He has been honored by Duncan lately, and he does not want to ruin the "Golden opinions" that people have of him so soon after he has acquired them (1.7.36). He seems to be feeling a number of concerns, including concerns for his own soul. When Lady Macbeth insults his masculinity, though, calling him a coward and something less than a man, Macbeth relents, saying, "I am settled and bend up / Each corporal agent to this terrible feat" (1.7.92–93).
Later, he is incredibly remorseful after he murders Duncan, fearful because he "could not . . . pronounce 'Amen'" and worrying that he will not be able to sleep peacefully anymore because he has "'murder[ed] sleep'" (2.2.42, 2.2.48). However, he seems to have no trouble whatsoever killing Duncan's equally innocent chamberlains. Murder comes more easily to him the second time, and Macbeth is able to kill the grooms without even consulting or being prodded by his wife.
Macbeth reaches another turning point when he decides to order the murder of Banquo and Fleance without consulting his wife. He actually lies to Lady Macbeth about it, telling her to "present [Banquo] eminence" at the banquet that night, though he knows full well that Banquo will be dead by then and so will not attend (3.2.35). Moreover, he has been "keep[ing] alone" of late, rather than taking pleasure in her company and advice, as he always seems to have done (3.2.10).
Macbeth experiences another turning point after he goes to speak with the Weird Sisters for a second time. He seeks them out to learn more about his fate, and though he now believes that he is untouchable, he decides to murder Macduff's wife and children. He says,
From this moment,
The very firstlings of my heart shall be
The firstlings of my hand. And even now,
To crown my thoughts with acts, be it thought and
The castle of Macduff I will surprise (4.1.166–171).
He will no longer consider things so deeply. Macbeth vows that, from now on, as soon as he considers something, he will act on it. At this point, he has morally descended even further: he's gone from murdering grown men (e.g., Duncan, Banquo) to attempting to murder a young boy (Fleance), and now to murdering a defenseless woman and her several children (Macduff's family). He gets worse and worse.