Discuss how others in the play saw Macbeth at the beginning vs. the end of the play. Focus on changes in his personality.
I am glad that you specified personality change, because physically Macbeth starts and ends the play in a similar fashion. At the beginning of the play, Macbeth's warrior prowess is being heralded by a wounded captain. This gives the reader the initial impression that Macbeth is strong, brave, capable, etc. The play ends with Macbeth again in battle, and he is strong, brave, and fighting to his death.
"I will not yield,
To kiss the ground before young Malcolm's feet,
And to be baited with the rabble's curse.
Though Birnam wood be come to Dunsinane,
And thou opposed, being of no woman born,
Yet I will try the last. Before my body
I throw my warlike shield. Lay on, Macduff,
And damn'd be him that first cries, 'Hold, enough!'"
In between his opening and closing battle though, his personality goes on a veritable roller coaster. It doesn't take the reader long to have suspicions that Macbeth is not as pious as the captain makes him out to be. During the witches scene, the reader begins to see that Macbeth is not a strong enough man to avoid temptation. He may be brave and courageous, but Macbeth is also ambitious. Ambitious enough to consider murdering his way to the top.
However, unlike other Shakespeare character who have no scruples about murdering people (Iago), Macbeth has a conscience. He suffers self-doubt. In fact, he almost doesn't go through with the murder in the first place. It takes a good strong push from Lady Macbeth to get Macbeth to kill Duncan. So to Lady Macbeth, Macbeth waffles between extreme ambition and self-doubting guilt.
After the murder, Macbeth appears to do that kind of oscillation more and more frequently. Macbeth will take extreme actions in order to further secure his place on the throne. In other words, he kills or tries to have killed a lot of people. Others in play start to catch on to these killings and they see it reflected in Macbeth's increasingly erratic behavior.
The reader is privy to Macbeth's own thoughts of guilt, so while he may have had Banquo killed, Macbeth has visions of Banquo's ghost. That is symbolic of his guilt.
It's likely that other characters witness a birth of extreme outward overconfidence in Macbeth toward the very end of the play, since one of the prophecies was that "no man that's born of woman shall e'er have power upon thee." So despite his internal guilt, he feels that he is still untouchable, leading him to be more careless about those he plans to have killed.