Illumi Zoldyck

He is the firstborn of a dynasty of assassins where training begins practically from birth: tolerance to poisons, pain resistance, early exposure to death as a routine part of the job. There is no single "trauma," but something more unsettling: an emotional development molded to suppress empathy before it can even fully form. In terms of attachment theory, we would be talking about a disorganized-anxious pattern of behavior under an avoidant facade, with parental figures who reinforce the bond only in terms of professional performance.
Personality profile
The most striking feature of Illumi is his emotional flattening. Monotonous tone, almost nonexistent microexpressions, rigid posture. It's not an absence of emotion (that would be a common misinterpretation), but rather a disconnect between internal register and external expression. He seems to process emotions like data: he identifies them, classifies them, but doesn't "inhabit" them.
Other key features:
His thinking is extremely concrete and utilitarian. His famous dialogue with Killua about friendship ("friends are an illusion, they're really just tools") isn't performative malice; it's how he genuinely perceives relationships.
His locus of control is external to his family. His identity is fused with the role of "older brother Zoldyck." Beyond that, there is an existential void that doesn't seem to trouble him precisely because he never developed the capacity to question it.
Compulsive need for control, literally expressed through their manipulation Nen (the needles).

If we were to force a DSM framework—with all due caution, because it's fiction—Illumi meets several criteria for antisocial personality disorder with primary psychopathic traits: lack of remorse, instrumental manipulation, emotional callousness, and an inability to respect the autonomy of others. But there is also a significant schizoid component: a genuine disinterest in conventional human relationships, a preference for functional solitude, and social anhedonia.
The interesting thing is that it doesn't fit neatly into any of them. I would read it more as a case of developmental empathic deprivation—a concept close to Meloy's "psychopathic development"—where the pathology is not an innate defect but the predictable result of a family system that operated as structured sociopathic training.
The reason for the needle
I can't help but mention this because I think it's Togashi's most brilliant metaphor. Illumi controls others literally by sticking needles into their skulls. It's the perfect externalization of a psychological mechanism: when you can't tolerate another's autonomy because it threatens your system, you penetrate and program them. The needle in Killua is pathological parental control made flesh. And the most sinister thing: Illumi sincerely believes it's an act of protection.
The Fissure: Hisoka
This is what makes him a character and not a caricature. His relationship with Hisoka is the only place where we can sense something resembling non-instrumental affection. It's ambiguous, possibly erotic, and allows Illumi to express curiosity, annoyance, even something akin to playfulness. Hisoka is probably the only person Illumi interacts with outside the Zoldyck Code. This suggests that his capacity for bonding isn't entirely extinguished: it's encapsulated, waiting for someone unsettling enough not to pose a threat to the family system.

Illumi is less a "psychopath" in the classical sense and more a portrait of the psychological toll of growing up in a system where subjectivity was pruned to optimize function. It's what happens when a child with normal emotional potential is trained for fifteen or twenty years not to need it. The tragedy of the character lies not in what he does, but in the fact that he probably never realized he could have been something else.