Final answer:
The line is spoken by Macbeth in Act 5, Scene 5, indicating his desensitization to violence after committing numerous murders.
Step-by-step explanation:
The quote "I have supped full with horrors; direness familiar to my slaughterous thoughts cannot once start me" is said by Macbeth in Act 5, Scene 5 of Shakespeare's play. This line reflects Macbeth's numbness to violence and terror, suggesting that he has become accustomed to such dreadful thoughts that they no longer have any impact on him. This moment in the play indicates Macbeth's descent into desensitization after his many murderous acts.