The King's Deadly Paranoia
You've probably felt paranoid about something before, but Macbeth takes it to a whole new level. Now that he's king, he's absolutely terrified of losing his power, so he decides Banquo must die. Why? Because Banquo was there when the witches made their predictions and is getting suspicious.
Here's where it gets interesting - Macbeth, supposedly Scotland's greatest warrior, can't even do the killing himself. Instead, he hires assassins to murder both Banquo and his son Fleance. When the murderers report back with "There's blood on thy face," Macbeth doesn't feel guilty at all - he actually congratulates them, calling them "thou art the best cut-throats."
But Macbeth's brief confidence crumbles when he learns Fleance escaped. He says "I had else been perfect" and describes himself as "whole as marble" - suggesting his plan (and his manhood) was flawless but is now cracked. The alliteration in "cabin'd, cribb'd, confined" shows how trapped he feels by his own fears, transforming from the mighty "eagle" and "lion" into hunted prey.
Key Insight: Notice how Macbeth calls Banquo a "serpent" and Fleance "the worm" - he's trying to convince himself that murdering his best mate was justified!