How does this quote from Macbeth (3.2.13-15) relate to power?"We have scotched the snake, not killed it: She'll close and be herself, whilst our...

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.