In the "Genre" section, we explained how Titus Andronicus is a special kind a tragedy known as a revenge tragedy. Since all revenge tragedies feature a big bloodbath at the end, we're not really surprised when Titus kills Demetrius and Chiron, then bakes them into a scrumptious pie that he serves to Tamora at a fancy dinner banquet. We're also not surprised by the string of stabbings that occur immediately after "Chef" Titus announces his secret ingredients. (Titus stabs Tamora, then Saturninus stabs Titus, then Lucius stabs Saturninus.) By this point the stage looks like one of Jackson Pollock's famous "splatter" paintings, and the outlook for Rome is not so great, since just about all of its most important political leaders are dead.
But, there may be some hope for Rome. When Marcus looks around and sees how Rome has been torn apart by Titus and Tamora's little blood feud, he declares "O, let me teach you how to knit again [...] These broken limbs again into one body; / Lest Rome herself be bane unto herself" (5.3.2). So even though the play has been filled with unspeakable violence and destruction, Shakespeare is now interested in how unity can be restored after civil strife. (This is a common theme at the end of all Shakespearean tragedies, by the way.)
When Marcus makes his big "let's put Rome's broken body back together again" speech, Titus Andronicus comes full circle, because the play began with the idea that Rome is not united as one whole "body." Think back to Act 1 when Bassianus and Saturninus were on the brink of starting a civil war because they both wanted to rule Rome. When Titus refused to be emperor, he said there was a "better head" that would "fit" Rome's "glorious body." Fast-forward to final scene of Titus Andronicus: Lucius is chosen as Rome's new head honcho, and he somberly promises to "heal Rome's harms" (5.3.7).
At the same time, Shakespeare can't resist cracking yet another grisly joke. Just as Marcus tells us that Rome is a "broken" body in need of mending, we can't help but notice all the dead bodies (and body parts) that litter the stage. So even though all the characters are standing around delivering serious speeches, it sort of feels to us like Shakespeare is pulling our leg.
We also want to point out that, even though Lucius is supposed to be the guy who will "heal" Rome and put an end to all the violence, Lucius turns around and orders that Aaron be buried and left to starve: "Set him chest deep in earth and famish him / There let him stand and rave and cry for food" (5.3.9). He also warns that anybody who tries to help Aaron will be put to death. We don't know about you, but this doesn't exactly sound like Lucius has put an end to the vicious cycle of revenge.