After Duncan's murder, Macbeth has been crowned king. However, instead of bringing Macbeth contentment, this new position has made him fearful and anxious. Not only is Macbeth consumed with guilt, but he is also fearful that this position can be taken away from him as easily as he took it from Duncan. Neither Lady Macbeth nor Macbeth has been happy since the death of Duncan. Their murderous deed did not have the outcome they expected. As Lady Macbeth declares
Tis safer to be that which we destroy
Than by destruction dwell in doubtful joy.
Macbeth seems to echo this sentiment with the quotation that you have chosen. He believes that in killing the guards and placing the blame on Duncan's two sons that he has only wounded ("scotched") the threats ("the snake") to his future as king. amd that these threats will in time grow even more dangerous to his reign by potentially exposing the crime ("our poor malice") he and Lady Macbeth committed.
In order to maintain his power as king, Macbeth feels the need to remove all threats. He targets Banquo as the most dangerous threat to his power. Banquo is wise and brave. Banquo heard the witches' prophecies and may suspect Macbeth of murdering Duncan, and Macbeth believes that Banquo is courageous enough to act on his suspicions. Further, the witches' predicted that Banqo's sons will be kings. Macbeth wants to ensure that he and his descendants remain in power, not Banquo's. Thus, Macbeth decides to make arrangements to have Banquo murdered.