I need help with an essay about kingship and tyranny in Macbeth. I need quotes that prove Macbeth is a tyrant.

Macbeth reveals himself as a tyrant in many ways, and ordering the killing of Banquo and his young son, Fleance, certainly proves that Macbeth enjoys absolute power and is quite ruthless, especially in an attempt to hold on to his power. Because Macbeth is concerned that he has ruined his soul and weighted his conscience with the murder of Duncan only so that Banquo's issue can take his throne, he decides to kill them both....

Macbeth reveals himself as a tyrant in many ways, and ordering the killing of Banquo and his young son, Fleance, certainly proves that Macbeth enjoys absolute power and is quite ruthless, especially in an attempt to hold on to his power. Because Macbeth is concerned that he has ruined his soul and weighted his conscience with the murder of Duncan only so that Banquo's issue can take his throne, he decides to kill them both. He convinces two murderers that Banquo is their enemy, saying that he is responsible for their misfortunes. He charges them to kill Banquo while he travels, as well as

Fleance, his son, that keeps him company,
Whose absence is no less material to [Macbeth]
Than is his father's, must embrace the fate
Of that dark hour. (3.1.154-157)

Thus, he arranges the murder of a child, callously and with no concern for anyone but himself and his own ambition and pride. It won't be the last time.

After his failed dinner party, when Banquo's ghost attends but Macduff does not, Macbeth tells his wife that he "keep[s] a servant fee'd" in all of his nobles' homes (3.4.164). So, he pays a servant in each house to spy for him and report the comings and goings back to him. This kind of surveillance is typical of a tyrant. Further, after the murder of Banquo and escape of Fleance, Lennox flat out calls Macbeth a "tyrant" when speaking to another Lord (3.6.25).  

Further, ordering the murder of Macduff's wife and children proves that he is a tyrant. Upon learning that Macduff has fled the country, he says,

The castle of Macduff I will surprise,
Seize upon Fife, give to th' edge o' th' sword
His wife, his babes, and all unfortunate souls
That trace him in his line. (4.1.171-174)

There is no reason to kill Macduff's family; Macbeth has nothing to gain politically or otherwise by the murder of a woman and her children. He only orders the killings to injure Macduff and to prove his own power. It is ruthless and incredibly cruel behavior indicative of his tyranny.