"Aaron the Moor" is Tamora's secret lover and the diabolical mastermind behind the plot to destroy the Andronicus family. He's also, in our humble opinion, the play's most fascinating character.
When, at the play's end, Marcus looks around at the bodies that litter the stage and declares that Aaron is the "Chief architect and plotter of these woes" (5.3.3), we have to agree. Aaron convinces Demetrius and Chiron to rape Lavinia (2.1), he's responsible for framing Martius and Quintus for the murder of Bassianus (2.1), and he even convinces Titus to lop off his own hand (3.1).
OK, we know Aaron causes a lot of suffering – but why does he do it? Because he wants to help his lover, Tamora, get revenge against Titus? Maybe, but Aaron never comes out and says as much. There's got to be some better explanation. We could argue that Aaron's only motive for going after the Andronicus family is that he just likes to be bad. Check out what Aaron says when Lucius demands to know whether Aaron is sorry for his "heinous deeds."
Ay, that I had not done a thousand more.
Even now I curse the day--and yet, I think,
Few come within the compass of my curse,--
Wherein I did not some notorious ill,
As kill a man, or else devise his death,
Ravish a maid, or plot the way to do it.
Accuse some innocent and forswear myself,
Set deadly enmity between two friends,
Make poor men's cattle break their necks;
Set fire on barns and hay-stacks in the night,
And bid the owners quench them with their tears.
Oft have I digg'd up dead men from their graves,
And set them upright at their dear friends' doors,
Tut, I have done a thousand dreadful things
As willingly as one would kill a fly,
And nothing grieves me heartily indeed
But that I cannot do ten thousand more. (5.1.10)
Yikes! Before Aaron was brought to Rome as a prisoner of war, he enjoyed a lengthy career wreaking havoc on the lives of others. Sounds like a psychopath, don't you think? He also sounds a lot like another notorious Shakespeare villain, Iago, whose motives for destroying Othello and Desdemona are completely suspect and murky.
Yet later, when Aaron lashes out at the nurse for calling his dark-skinned child a "joyless, dismal, black and sorrowful issue" that should be "christen[ed] with a daggers point" (4.2.6), we can see that Aaron's behavior may be a response to the way the Romans and Goths treat him as loathed outsider. This seems especially true when he shouts at the nurse, "'Zounds ye whore! Is black so base a hue?" (4.2.11).
As we've seen, Titus Andronicus makes a very big deal out of Aaron's blackness, and Shakespeare draws on some 16th century stereotypes in its portrayal of Aaron's character. For example, Aaron's blackness is synonymous with evil throughout the play. He's referred to as a "barbarous" Moor more times than we can count. (FYI: for Shakespeare, the terms "Moor" and "blackamoor" generally refer to any black character.) Also, Aaron displays the kind of hypersexuality so often associated with black characters in Elizabethan literature.
The interesting thing about Aaron is that he simultaneously embraces and defies these stereotypes. The clearest example of this is when Aaron declares he loves being a villain:
Let fools do good, and fair men call for grace.
Aaron will have his soul black like his face. (3.1.4)
Aaron is equally rebellious when Tamora gives birth to his illegitimate love child. Check out what he has to say when he learns he's a dad:
Well, God give her good rest! What hath he sent her?
Why, then she is the devil's dam; a joyful issue. (4.2.8)
When the nurse calls Aaron's dark-skinned child a "devil," Aaron doesn't disagree. Instead, he embraces the nurse's racist remark and turns it into a positive, insisting that his little "devil" is a "joyful issue" ("issue" meaning something that comes out of something else, i.e. a baby), and that his lover, Tamora, must be the devil's "dam" (mother).
While Aaron is very clearly the villain of the play (and we could even say he's one of the most evil villains in all of Shakespeare), we do see a slight glimmer of humanity in him. Aaron, after all, seems like the only parent who's willing to put his child first. In a world where Titus kills two of his children onstage for no good reason, and Tamora orders Aaron to "christen" her baby with a "dagger's point," Aaron's defense of his newborn baby seems almost admirable:
Stay, murderous villains! will you kill your brother?
Now, by the burning tapers of the sky,
That shone so brightly when this boy was got,
He dies upon my scimitar's sharp point
That touches this my first-born son and heir! (4.2.15)
What's more, Aaron cuts a deal with the Romans in order to save his child's life, promising to reveal the entire revenge plot if the Romans will allow his child to live.
However promising Aaron may be as a father, the Romans see fit to punish him after he's captured. When Lucius announces that Aaron will be buried up to his neck and left to die (raising the question of Rome's so-called civility), Aaron remains defiant to the very end.
O, why should wrath be mute, and fury dumb?
I am no baby, I, that with base prayers
I should repent the evils I have done:
Ten thousand worse than ever yet I did
Would I perform, if I might have my will;
If one good deed in all my life I did,
I do repent it from my very soul. (5.3.1)