At the beginning of the play, Tamora is a sympathetic figure. As a war prisoner, she is paraded through the streets of Rome and is helpless when her eldest son is sacrificed by the Romans. But ultimately, Tamora is an antagonist in the play, because she goes after the Andronicus family without mercy. Tamora may have a good reason to seek revenge against Titus, but she loses all sympathy when she encourages her sons to rape Lavinia and then runs off to steam up the windows with Aaron.
At the end of the play, Marcus looks around at all the destruction and announces 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). In short, Aaron is the cause of a whole lot of suffering.
The thing that separates Aaron from most other literary antagonists is that he doesn't really have any just motive for hurting the Andronicus family. He just likes to be bad (which makes him a lot like the infamous villain Iago in Shakespeare's Othello). For example, when Lucius asks Aaron if he's sorry for his "heinous deeds," Aaron famously replies, "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" (5.1.10). Yikes!