Why didn't Macbeth kill Banquo himself? Why give the job to those two murderers?

These are interesting questions. There are several possible answers. In the first and most important place, Shakespeare was trying to maintain some degree of audience sympathy for Macbeth because this was his tragic hero. He couldn't show Macbeth going around murdering people and still maintain any sympathy for him. We feel little enough sympathy for Macbeth in the end as it is. The only good thing to be said about him is that he is extremely courageous. He even challenges Fate itself and goes down fighting Fate, which he finds is invincible.

Furthermore, Macbeth is not really a murderer. He has had enough of cold-blooded murder when he kills Duncan in his sleep, as we can see in the aftermath. When his wife tells him he must go back to Duncan's chamber and smear the faces of the drugged grooms with blood from the two daggers, he replies:

I'll go no more:
I am afraid to think what I have done;
Look on't again I dare not.          (2.2)

So his wife has to take the daggers and go back to do the grisly job for him. She is one of the first to do his dirty work. When Macbeth thinks of killing Banquo it is natural for him to find someone else to do it. He doesn't mind killing men in battle, but he doesn't like committing villainous murders. We can say that for him too: he is not much good as a murderer. He delegates Banquo's and Fleance's murders to two men who are joined by a third murderer just before the assault takes place. And later when he has a number of soldiers slaughter Macduff's family and everyone else in Macduff's castle, he is getting someone else to do his dirty work. This is partly because he is squeamish about the criminal kind of killing and also because Shakespeare didn't want to make him look any worse than he already was. We are supposed to feel some pity for Macbeth at the end, when he is all alone and everybody hates him, and he is totally depressed.

In addition to these reasons, there is the fact that Macbeth becomes king almost immediately after Duncan's assassination. As king, Macbeth has more power. He can kill anybody he wants to--but he has to be concerned about public opinion. When he has Banquo killed at some distance from his castle, he makes sure that he has a good alibi. He is hosting a big banquet. How could he have killed Banquo and tried to kill Fleance? But when he has Macduff's wife and children murdered, he no longer seems to care about what people think. In fact, he does this openly in order to set an example. He wants to show what can happen to any of his followers who, like Macduff, desert him. 

Macbeth might have another reason for not planning to kill Banquo himself. Banquo is a warrior. He would not be as easy to kill as an old man sound asleep in his bed. Macbeth might feel that he would need help. After all, there are two people involved, Banquo and his son Fleance. Fleance might get away while he was fighting with the boy's father--which is pretty much what happens. Banquo and his son may stay at Dunsinane for another night, but we can be sure that Banquo would be wide awake and have his door securely bolted and barricaded. We see in Act 2, Scene 1 that Banquo keeps his sword with him while he is in Macbeth's castle. Early in the scene he tells Fleance:

Hold, take my sword.

He does this because Shakespeare wanted to show his audience that he has a sword. Then when Macbeth enters, Banquo says:

Give me my sword!

Banquo knows he is in danger because he knows--just as we know--that Macbeth cannot like the idea of Banquo's descendants forming a long line of Scottish monarchs. Banquo is also sure that it was Macbeth who killed Duncan, even though he managed to pin the blame on Malcolm and Donalbain. Banquo knows Macbeth is a very dangerous man, and he doesn't like being a guest in his castle.

Shakespeare may not have wanted to write yet another murder scene in which Macbeth goes creeping down the corridors in the dark. It would be too repetitious. The playwright had wrung about as much emotion out of the first murder as he could expect to get. He needed some variety. A writer has to keep changing things in order to hold the interest of his reader or his audience. This is one of the secrets of good creative writing.