Why does Macbeth hire murderers?

Since this play is intended to be the tragedy of Macbeth, Shakespeare does his best to preserve some slight degree of audience sympathy for him. It is to be noted that, although Macbeth is responsible for the murders of Duncan and Banquo, the attempt to Fleance's life, and the slaughter of Macduff's whole family along with every other unfortunate in the castle, Macbeth himself is never actually shown doing any of these evil deeds. He murders Duncan offstage--and then he even sends his wife back to return the grooms' daggers and smear their faces with blood. He is too squeamish to look at what he has done to the King. Then he sends agents to attack Banquo and his son Fleance. When confronted by the ghost of Banquo at the coronation banquet, Macbeth tells him:
Thou canst not say I did it: never shake
Thy gory locks at me. III.4
Macbeth seems to think he isn't guilty of murder so long as he wasn't present when it occurred.
Then in Act IV.2 we see a force of soldiers invade Macduff's castle and murder Lady Macduff and her innocent little boy. Macbeth must have sent them, but we see nothing of Macbeth himself. He is doing everything by agents. It is possible that Shakespeare is tying to keep the audience from completely hating this man so that they can still appreciate his fall from brave warrior to cruel tyrant.
Macbeth even remains somewhat defiant at the end. He is not fighting Macduff so much as he is fighting against the laws of the universe, against the invincible hand of fate, against God himself.
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!” V.8