Macbeth in Act I is very different from the Macbeth we see in Act V. Macbeth in Act one has no intention to kill the king. When he hears from the witches that he is going to be king, he wonders aloud how that could be when Duncan and Duncan's sons are all living.
It isn't until he speaks with Lady Macbeth about it that he gets the idea to kill Duncan at all. It is Lady Macbeth's convincing that gets him to take action. She knows that she must do the plotting as she claims that Macbeth is Lady Macbeth herself characterizes Macbeth by saying that he is too "full o' the milk of human kindness." Looking at Macbeth's soliloquy in Act 1 Scene 7, he argues first that he has loyalty to Duncan his king and as such he should always protect him. Secondly, as his host it is his duty to provide food, shelter and help to him. Finally at the end of the soliloquy he admits
"I have no spur
To prick the sides of my intent, but only
Vaulting ambition, which o'erleaps itself
And falls on th' other" (1.7.25-28).
This quote shows that without Lady Macbeth he lacks the "spur" or motivation to kill Duncan at all. This is just the beginning of Macbeth's murders though. Once Duncan is dead, and Macbeth has recovered from guilt, he moves onto take out his other "enemies."
By act three, the first enemy that Macbeth identifies is his close friend and fellow soldier Banquo. The witches identified that Banquo's sons would be king some day, so Macbeth decides to kill Banquo as a precautionary measure. He also sends the murderers after Banquo's son too.
After the appartitions in act four, Macbeth feels secure about his immortality, but still sends murderers after Macduff and Macduff's family. Macduff's family includes a small boy and Lady Macduff who both die mercilessly at the hands of a murderer.
No longer does Macbeth question whether or not it is moral to kill or whether or not he should even feel guilt about what has happened. He has significantly changed from the man that Lady Macbeth influenced from the beginning. He no longer even consults her on the major life decisions he makes for himself, their family and the country.