Terra's stagnation and Gaia's dynamism
Once his plan to overthrow Garland teeters on the edge after his thwarted attack on Alexandria, Kuja drops his sinister theatricality and begins to show fits of juvenile temper, swinging between hysterics and snapping unprovoked at his subordinates. Until he grasps the source of Trance, an esoteric power once barred to him, he scrabbles for purchase by using everything and everyone within his reach to challenge his powerlessness under Garland's thumb. It's only after he successfully manipulates Zidane's party into opening a path to Terra by taking advantage of Hilda's rescue that he calms again, giddied by the prospect of finally wresting control from his creator.
Although a lover of theatre who realizes his schemes on Gaia like a playwright, his temperamental behavior upon losing control is Kuja at his most honest. Due to the nature of his creation, he never experienced a proper childhood during which he could grow and manage his emotions. Because Terra is a stagnant planet populated solely by Genomes and monsters, Kuja mostly knew only himself for the first eight years of his existence. This left him comfortable with solitude while inflating his self-importance as the only Genome in Bran Bal with a soul to call his own, giving rise to his pride in present day. Concurrently, the absence of meaningful interactions saw that he had no opportunities to reflect on his character outside of arguing with Garland about the meaning of his existence, which exacerbated matters.
Kuja serves as an excellent foil to Zidane, because he opposes his brother in every feasible way, right down to the complementary colors of purple and green in their designs. He's tall where Zidane is relatively shorter; the white (or silver) to his gold; the solitary to the companion; the selfish to the kind; the Terran to his Gaian. He even takes the souls of departed Gaians, many of whose deaths he caused, to achieve Trance in the same parasitic way Terra intends to subsume Gaia. The tragedy of Kuja's character comes from Zidane's understanding that, had they been in one another's shoes, they would've ended up like each other, as Zidane became the person he is now through his upbringing on the young and dynamic Gaia. By abandoning his loathsome rival on an alien planet, Kuja ironically gave Zidane the kind of life he desires the most, where the latter is free to abide by or leave the Tantalus Theater Troupe when it suits him while the former remains bound to Garland's authority.
Kuja's wish to rule over an eternal kingdom is, at the most fundamental level, a metaphor for autonomy. When he kicks Garland off the ledge to his death on Terra, he believes he's at last attained the power to truly live his own life as he pleases. Garland, however, reveals a truth that makes Kuja's freedom devastatingly short-lived.
Garland: Do you think a defect like you could last forever . . . ?
Kuja: . . . What? What do you mean!?
Garland: I built you to last only until the worthy Genome, Zidane, grew. It was too dangerous to let you last any longer than that.
Kuja: What are you saying . . . ?
Garland: There's a limit to your life . . . You'll be dead soon . . . Even as I die, you'll have died without ever leaving your mark on the world . . .
After twenty-four years of believing himself eternal and better than his Gaian counterparts, Kuja's self-esteem shatters upon realizing that he's no better than the black mages, who possess great power yet an inversely short life span. The fear that he'd lived with all his life and just released comes roaring back, and he throws what boils down to a temper tantrum by destroying Terra.
Trance is an untapped power granted to a select few on the planet, unlocked by a surge of emotion like anger. While Zidane and company are all able to utilize Trance naturally, Kuja's arrested development due to his Terran origins meant his emotions were too shallow. Garland had created Zidane as a newborn, because he'd assumed his first angel of death would never be able to reach that peak. Yet, for how Terran Kuja is all throughout FINAL FANTASY IX, he proves upon artificially manifesting Trance that he's actually more Gaian, and therefore dynamic, at his core.
In Trance, Kuja deceptively appears to be more Terran than ever with a full body of red hair and the reveal of his once hidden tail. But there's something truly remarkable he accomplishes for one who was born into stagnancy: Kuja grows—literally. His height increases from approximately 214 cm (7') to 226.3 cm (7'5"). That's about five whole inches! Even his leitmotif, which went from "bad vibes only" to a more recognizable homage to "Slumber of Ancient Earth" in "Wicked Melody," swells to a turbulent crescendo in "The Darkness of Eternity."
In order to grow a new identity, the old self must first die. For Kuja, the birth of his nascent self begins with the death of his superficial narcissism. Only once he's been stripped of the thing he holds dearest—his very life—does he show his true face to the world. He admits to his lifelong fear upon his defeat in the Crystal World, where his misplaced anger seemed to have stemmed not just from a sense of unfairness, but also the newfound dread of dying alone if he must die, marking the beginnings of a paradigm shift.
Kuja: What comedy! Zidane, isn't it hilarious?! I'll die just like the black mages I so despise! I single-handedly brought chaos unto Gaia, but in the end, I'm nothing but a worthless doll!
Zidane and company fight Trance Kuja in the Crystal World, where he attempts to end all of creation, on March 8th. After his defeat, he casts Ultima as a desperation move before falling through the fabric of time and space, then down the Iifa Tree until he hits a patch near the bottom. The party, meanwhile, battle the embodiment of death summoned by Kuja's despair through the 9th. It's not until the 10th that they return to Gaia. In other words, Kuja spends an entire day in the basin of the Iifa Tree, dying a slow and lonely death with nothing but his own thoughts for company, during which he comes to several epiphanies—chiefly, that he'd gone about everything wrong. In coming to terms with his mortality and acknowledging his innumerable crimes, he reflects and truly grows for the first time.
Kuja: . . . I don't deserve to live after all that I've done. I'm useless to this world.
Zidane: No one's useless . . . You helped us escape, remember?
Kuja: . . . . . . After you guys beat me, I had nothing left . . . nothing more to lose. Then, I finally realized what it means to live . . . I guess I was too late.
The eyes are the window to the soul. While Kuja's hair is naturally red, his eyes are blue like the color of Gaia's moon. Poetically, he dies at peace atop Gaian soil on his own terms, having preempted Garland's time limit on his life span, after rectifying his fraternal sin of abandoning Zidane twelve years ago by teleporting him to safety from the Hill of Despair.