After each of Macbeth's killings, Macbeth seems to get mentally weaker. Give proof.Evidence from the book should be given.
I wouldn't necessarily say that Macbeth was mentally strong to begin with. However, to address your question: after killing Duncan, Macbeth gets skittish. He reacts nervously to everything and seems almost unhinged: Whence is that knocking? How is't with me, when every noise appals me?
After Banquo's murder, Macbeth begins to hallucinate. He sees Banquo's ghost in the hall and is tortured by his own guilt:
Avaunt! and quit my sight! let the earth hide thee!
Thy bones are marrowless, thy blood is cold;
His guilt drives him to seek out the witches again and to gain more information to obsess over. This drives him to then attack Macduff's family. After these murders, Macbeth loses rational thought. He orders his remaining men back and forth, confusing and concerning them. He reacts little to the death of his own wife. He charges into a battle he knows he will lose, having admitted that he is suicidal and life doesn't matter. Like his wife, Macbeth has "lost touch" with reality.