Why does Macbeth choose murderers instead of soldiers?

Macbeth uses soldiers to carry out the atrocity at Macduff's castle, but he uses three peasants to perform the murder of Banquo and to attempt to murder Banquo's son Fleance. This seems to suggest that Macbeth grows increasingly careless about preserving a good reputation. In fact, he apparently wants all of Scotland to know he was responsible for the slaughter of Macduff's family and servants because he has decided that he has to rule by fear and wants to set a memorable example. 

When he has Banquo and Fleance ambushed in the dark by anonymous men, Macbeth does not want to be suspected of having arranged it. He thinks he can get away with the murders just as he got away with the murder of King Duncan. If he ordered soldiers to do the job, it would be impossible for him to keep his complicity a secret. We see in the banquet scene in Act III, Scene 4 that the first thing Macbeth says when he recognizes Banquo's ghost occupying his place at the table is:

Thou canst not say I did it. Never shake
Thy gory locks at me.

He intended to get away with Banquo's murder. That was why he incited two commoners who had private grudges to do his dirty work and would have to keep his secret to avoid being executed for their crime. But the appearance of Banquo's ghost at the banquet unnerves Macbeth and he gives himself away to all the assembled nobles. This apparently was what Banquo's ghost intended. After that Macbeth is disgraced. In one soliloquy towards the end of the play he acknowledges that, although he has gained the title, power, and riches he sought, they haven't given him any satisfaction.

I have lived long enough. My way of life
Is fall'n into the sear, the yellow leaf,
And that which should accompany old age,
As honor, love, obedience, troops of friends,
I must not look to have; but, in their stead,
Curses, not loud but deep, mouth-honor, breath,
Which the poor heart would fain deny and dare not.   (V.3)

He had wanted to be a king like Duncan who governed by love, kindness, justice and wisdom; but his subjects' hatred forced him to become a tyrant instead. His tyranny caused many subjects to flee to England. The disorder this exodus was creating in England was King Edward's prime motivation for sending a large army to unseat Macbeth and replace him with Malcolm.