In Macbeth, Act III, Scene 3, what causes the murderers to leave the job only half done?
The two murderers who originally spoke with Macbeth are joined unexpectedly by another man who is designated as the Third Murderer. The original two believe that Macbeth sent a third one because he didn't trust them to do the job right. However, it would appear that having three assassins involved in the actual killing only creates confusion because they get in one another's way. The only lighting is from the torch carried by Fleance, and this torch is knocked out in the scuffling, thus enabling Fleance to escape in the dark while his father Banquo is being stabbed to death and crying
O treachery! Fly, good Fleance, fly, fly, fly!
Thou mayst revenge--O slave!
In Act 3, Scene 4 the First Murderer has to report to Macbeth that they succeeded in killing Banquo but let his son escape. Macbeth is foiled and frustrated. He says:
Then comes my fit again. I had else been perfect,
Whole as marble, founded as the rock,
As broad and general as the casing air.
But now I am cabined, cribbed, confined, bound in
To saucy doubts and fears.