When MacBeth exclaims "we have scorched the snake, not killed it" he is referring of course, to Banquo and also really anyone who is a threat to his power and his crown. He thinks of Banquo in this way because of the witches' prophecy that he will produce kings but not be one himself. MacBeth unjustifiably refers to Banquo like this because he has identified Banquo as a threat that could, as a snake can, lurk in the underbrush and strike him when he least expects it. It is an ironic use of the image, since it is MacBeth who really is the "snake."
I'm not sure if you have the correct lines for your final question. Are you referring to when MacBeth refers to his mind as being "full of scorpions"? Qualify the lines, then I can add further insights.