Hecate will give Macbeth protection from death. Macbeth jealous of Duncan because he is now secure in death.
The scene starts with Hecate being annoyed with the witches because they didn’t invite her to cause mischief with Macbeth. She then goes on to come up with a plan to offer him immortality. Earlier in the play, Macbeth said “Duncan is in his grave. After life’s fitful fever he sleeps well. Treason has done his worst; nor steel nor poison, malice domestic, foreign levy, nothing can touch him further” exclaiming how envious he is that Duncan is now free of life’s troubles and can rest easy, whereas Macbeth has all this guilt and anxiety now that he is king.