In Act 1, Macbeth is just beginning to consider the possibilities of his becoming king. After Duncan gives Macbeth the title "Thane of Cawdor," he mentions that Malcolm is to be the next king. Having been shown that at least one of the witches' prophecies has come true, Macbeth really considers going after the crown and therefore sees Malcolm as an obstacle. However, Macbeth is conflicted about committing any crimes.
First, as I am his kinsman and his subject,
Strong both against the deed; then, as his host,
Who should against his murderer shut the door,
Not bear the knife myself. Besides, this Duncan
Hath borne his faculties so meek, hath been
So clear in his great office, that his virtues
Will plead like angels, trumpet-tongued against
The deep damnation of his taking-off, (I.vii.13-20)
Macbeth thinks it would be wrong to kill Duncan and he fears his own damnation if he does so. Lady Macbeth criticizes Macbeth and then encourages him to go through with the murder.
In Act 3, Scene 1, Macbeth is much more assertive and he confidently orders the murderers to kill Banquo and Fleance, this time without the direction or encouragement of his wife. In contemplating and discussing Banquo's murder, Macbeth has no moral conflicts as he did with Duncan. Macbeth notes that if the weird sisters' prophecies are true, then Banquo's descendants will be kings. In order to protect his lineage, Macbeth has no problem with killing Banquo:
If't be so,
For Banquo's issue have I filed my mind,
For them the gracious Duncan have I murdered,
Put rancours in the vessel of my peace
Only for them, and mine eternal jewel
Given to the common enemy of man (III.i.65-70)
Macbeth was conflicted about killing Duncan but at this point in Act 3, he has accepted that he's essentially sold his soul ("eternal jewel") to the devil ("enemy of man") in order to pursue and sustain his reign. At this point, Macbeth easily decides to defile ("file") his mind and plan Banquo's murder.