What is the significance of the following quote from "Macbeth"? (act 3, scene 4) The time has been/that when the brains were out, the man would...

Macbeth is at the banquet and he has just seen the ghost of Banquo. He is perturbed because, he says, it used to be that when you killed a man, he would just die, and that would be the end of it. This is significant in part because it is Macbeth's guilt that makes Banquo's ghost appear.  In the past, Macbeth killed only for his country and had no reason to feel guilty.