When Macbeth hears the women crying within the castle, he asks what he has heard. Upon hearing that "[i]t is the cry of women," he responds:
I have almost forgot the taste of fears:
The time has been, my senses would have cooled
To hear a night-shriek, and my fell of hair
Would at a dismal treatise rouse and stir
As life were in 't. I have supped full with horrors.
Direness, familiar to my slaughterous thoughts,
Cannot once start me.
Macbeth is not only not horrified by the "night-shriek" he hears, he is very aware that it does not frighten him. It affects him not at all, whereas there was a time when such a sound would have startled him so deeply that he would have experienced a physical reaction to it. Macbeth has been hardened and dehumanized by his multiple murderous acts. He has "supped full with horrors" (a subtle allusion to Banquo's ghost appearing at the banquet in Act III), from Duncan's terrible assassination to the slaughter of Macduff's entire household. By the end of his brief but bloody reign, nothing horrible can frighten or horrify Macbeth.