Can someone please explain this metaphor from MacBeth?

After Macbeth kills Duncan, the king's two sons run away, putting suspicion of the murder on them, leaving Macbeth looking guilt-free. But Banquo starts to act distant and Macbeth begins to think Banquo might suspect him of the murder. So the "snake" is the the danger of being caught for the murder - "scotched" means "rendered temporarily harmless" - but they haven't gotten away clean - they haven't "killed" the snake..... they still have Banquo to worry about.