Demetrius and Chiron are savage, ruthless young men, acting as contrast to the more reserved, soft, sinister villainy of the likes of their mother or Aaron, traditionally played in productions as loud, borderline insane young delenquints. Unlike the other villains of the play, who for the most part simply plan everything and rely on their lessers to carry them out, or their own wits, Demetrius and Chiron have to have their evil plans handed to them by Aaron, and are then the ones that actually carry the evil out. Thus while they cannot be said to be as threatening as the ruthless sociopathy of Aaron or even their mother, it must be remembered that Demetrius and Chiron, even if they didn't come up with the idea themselves, are the ones that actually commit the murder of Bassianus and rape of Lavinia.
In Titus Andronicus
Demetrius, the older of the two, and Chiron, Tamora's youngest son, are brought prisoner to the Roman Army after having been defeated by the eponymous general, along with their mother Tamora, her secret lover Aaron the Moor, and their brother, Tamora's oldest son, Alarbus. To appease the Gods and spirits of his own fallen sons, Titus decides to have his remaining sons murder Alarbus as a sacrifice, ignoring Tamora's pleas for mercy. Demetrius and Chiron mourn the loss of their brother with her ("Was ever Scythia half so barbarous?" - Chiron, Act I Scene I), until Saturninus is wooed by Tamora, gaining them freedom, as Tamora plots revenge.
As Aaron plots how he can use the rise of power of the woman who loves him to his advantage, Demetrius and Chiron enter arguing over Lavinia, the lone daughter to Titus, who they are both madly in love with. Aaron sees this as an oppurtunity to not only help out Tamora in revenge, but also spread chaos and ruin with evil deeds. He seperates Demetrius and Chiron, who are about ready to kill one another, and suggests that they simply both have her by force - they simply rape her. Demetrius and Chiron gleefully plot the rape.
The following morning, as Titus and his family are on a hunt with Saturninus, Tamora and her sons as a way of peace, Demetrius and Chiron prepare themselves as Aaron and Tamora get everything ready ("Chiron, we hunt not, we, with horse nor hound, But hope to pluck a dainty doe to ground." - Demetrius, Act II Scene II). Lavinia and her new husband Bassanius come across Tamora and Aaron in the woods, becoming aware of Tamora's affair, and both scorn her for it. Aaron leaves to conduct the rest of his plan, and Demetrius and Chiron enter. Tamora tells them of how she has been scorned, and that if they love her they will take revenge. In sinister delight Demetrius and Chiron murder Bassanius, and prepare themselves to rape Lavinia, drawing it out to terrify her more. She begs for mercy, but Tamora just laughs and leaves Demetrius and Chiron to do what they will with her. They laugh at her and mock her, rubbing more salt in the wounds by cutting off her hands and slicing off her tongue so that she cannot communicate the rape to anyone. The two then flee, leaving her to be found by Titus's brother, her uncle Marcus. After the events of the play begin to unfold, Titus starts going insane on account of the assault on his daughter, and the murder of his two innocent sons, who Aaron framed for the death of Bassianus, and lost his hand as a result, Lavinia uses the books in the libarary to communicate to the rest of her family that she has been raped, and then uses a stick in between her stumps (or depending on the production, simply uses her toe) to write in the garden sand the name of her attackers, and it is finally revealed that Demetrius and Chiron were the ones that killed Bassianus and raped Lavinia, mutilating her in the process.
Meanwhile, Demetrius and Chiron are discussing the events with Aaron, until a nurse comes in and reveals that on account of Aaron and Tamora's affair, Tamora has given birth to a black child to which Aaron is the father. Knowing that if Tamora is found out of her affair she will die, and the shame that birthing a bastard child will cause her, Chiron and Demetrius want the child killed, ("And therein, hellish dog, thou hast undone. Woe to her chance, and damn'd her loathed choice! Accursed the offspring of so foul a fiend! - Demetrius, Act IV Scene II) as does the nurse (though her for no other reason than it is black). Aaron however kills the nurse and refuses to let any harm come to the child, and sends Demetrius and Chiron away to their mother as he plots what to do with the child.
Meanwhile Tamora and Saturninus have begun to suspect that Titus has gone insane and uses it as an oppurtunity to try stop Lucius's army against the corrupt empire. Tamora disguises herself as revenge, and brings with her Chiron and Demetrius, disguised as Rape and Murder ("Show me a murderer, I'll deal with him." - Demetrius. "Show me a villain that hath done rape, and I am sent to be revenged on him." - Chiron, Act V Scene II). Titus however is not quite as insane as the three villains give him credit for, only pretending to be and plays along, pretending to think they are Revenge, Rape and Murder, though knowing who it is in reality. Thus, he sends Tamora away, and captures Chiron and Demetrius with the help of his followers.
Titus approaches the two brothers, who by this point have been tied up and unable to talk, bringing Lavinia with him, he carrying a knife and her a basin. He boasts in a lengthly monolouge that he knows of their crimes, and to bring the cycle of revenge full circle he will murder them, and bake them into pies and feed them to Tamora. He slits Chiron and Demetrius's throats, letting Lavinia use the basin to catch the blood, and proceeds to bake them into pies which he then feeds to Tamora in a great feast meant as a peace dinner. He murders Tamora, and in revenge is murdered by Saturninus, who himself is murdered by Titus's lone remaining child Lucius, who becomes Emperor, promising to rule with wisdom and goodness.
Demetrius and Chiron, being princes of the "barbarous Goths" army act as foils to what Rome is meant to represent in honour, nobility and peace. The two brothers are childish, cruel, savage and can easily be portrayed on stage as insane, to the extent as wanting an incestual relationship with their mother, or even themselves (as they are portrayed in Julie Taymor's 1999 adaptation entitled Titus). They are hedonistic manchildren, willing to try kill each other over Lavinia, and only laugh at her and mock her after they are done raping and torturing her ("Write down thy mind, bewray thy meaning so, An if thy stumps will let thee play the scribe." - Chiron, Act II, Scene IV). A lot of how they are portrayed however comes down to how actors and directors wish to portay them, for the original text in itself offers little more than this.