Kuja was a villain done right in Final Fantasy. He was definitely a bad person, but a bit more believable. He is cringe-worthy personality points to a disorder:
Narcissistic Personality Disorder. People with narcissistic personality have an exaggerated sense of self-importance, are absorbed by fantasies of unlimited success, and seek constant attention. The narcissistic personality is oversensitive to failure and often complains of multiple somatic symptoms. Prone to extreme mood swings between self-admiration and insecurity, these people tend to exploit interpersonal relationships.
Everyone in the game sees Kuja as a narcissist. As a villain, Kuja is despicable. As a person, he’s irritating, obnoxious and characteristically ugly. Kuja has high unwarranted self-importance and is extremely arrogant. Whenever he’s on screen, he exudes a presence of extreme superiority over others. Kuja believes he’s too good for others. He’s also verbally harsh.
Kuja is narcissistic in contrast to Zidane’s humility. Kuja has an ego to match his power, and he lets his pride rule his head. Kuja once chose to let Zidane live even if he had a chance to kill him to prove himself superior. After discovering his destiny as a mortal, Kuja’s cultured personality turns into insanity, believing life should not be allowed to continue without him. He is unfazed by being wounded, shown when being attacked by Bahamut.
From the scenery in his Desert Palace, we can see he loves opulence. I’m sure his fantasies involve being desirable, attractive, successful and powerful. He congratulates himself on his own ideas, laughs at his own jokes, and [probably expected praise when he told Regent Cid’s wife his plans.
Kuja uses his elegance to hide his insecurities, and deep down he is conflicted by his existence. Kuja scorns the black mages, but also finds himself identifying with them. As explained in Final Fantasy IX Ultimania Kuja never experienced childhood, and thus his emotional growth is stunted, being unable to empathize with others or feel remorse for his actions. It appears that learning of his fate as a mortal and rebelling against this destiny allows Kuja to grow, and he comes to see at least some of the repercussions of his actions.
As far as toying with others, he convinces the Black Mages to fight on his behalf based on a lie that he can extend their life. Examples like these (including giving those he dislikes disparaging names like “rats” and “elephant lady”) make it clear he doesn’t recognize other’s feelings.
Kuja’s speech patterns are metaphorical and jovial; for example he refers to Garnet as a “canary”, and Brahne as “the elephant-lady”. His refined nature juxtaposes his role as a sadistic war-bringer. Kuja loves luxury, as evidenced by his home at Desert Palace.
That, and the fact that he destroys a world once he finds out he’s going to die very soon. Kuja won’t allow the world to exist without him. It’s very clear he considers himself more special and important than those around him. Towards the end of the game, he ends up gaining a heart to atone for his past crimes. But as far as learning how to have empathy, it’s relatively unknown.