Why is the following comment by macbeth in act v, scene 5 important?
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.

It is important for it shows just how far Macbeth had changed from the start of the play to the end, and that too, reflected in his own words.

Explanation:

In William Shakespeare's "Macbeth", the title character had been greedily acting for his own gains. And towards the end of the play, even he seems to acknowledge his change, his own indifference to the things he used to care about before.

In the lines from Act V scene v, Macbeth seems to imply that through all that he has done and seen, he seems to be unaffected by anything anymore. he is immune to all the torturous deeds committed, becoming indifferent to people's sufferings. The first part of the excerpt shows himself reflecting how he would have been affected by anything previously, but now, along with the greed and compelled by his wife to do deeds unfathomable, he had indeed changed. Now, he is left without any humane feelings, unaffected and indifferent to any suffering. This comment of his is important for it shows, in his own words, how he had changed for the worse.