[Spoilers] Zandik's life is one of if not the most tragic story in Genshin Impact. The unfairness of Teyvat's fate.
His fate and destiny were cursed from birth. He was born with the name "Zandik" which means heretic in Middle Persian. No matter what he did and how, his actions were always seen as hostile and bad.
Zandik actually did try to use his genius intellect to treat and help people suffering from illnesses. However, because his analytical methods were completely unorthodox and broke rigid societal taboos, his attempts to help were met with fear, disgust, and hostility. That's why he was treated as an outcast and a monster by his own home village. He was eventually chased out of his hometown with pitchforks and clubs.
When an 8 year old child is entirely rejected by society, viewed as inherently evil, and given zero emotional or societal support, the psychological defense mechanism is often to completely turn off empathy. He stopped caring about others because the world explicitly showed him that it did not care about him. He was just a child... 8 years old baby, who couldn't win against his destiny/fate. It's like a tragic story of Oedipus the King who's fate is completely subordinated to inevitable destiny. Every attempt to escape the prophecy paradoxically leads to its fulfillment even tho Oedipus wanted to do good, guided by noble principles. So, even if Zandik tried to do good, his methods were seen as something bad just because of his cursed, heretic fate.
Losing his compassion at age 8 marks the exact moment he realized that trying to heal people "traditionally" or empathetically was a dead end. He completely detached his emotions from his work, pivoting toward cold, clinical, and transactional egoism. He lost his compassion as a result of unimaginable trauma.
This age (8 years old) is incredibly significant because when Dottore made his segments, Segment 8 was the youngest batch he created. Segment 8 exists as a physical manifestation of the point where his humanity died and his detached, purely scientific mind took total control. That's literally a manifestation of his trauma.
Because fate shattered his life from birth, it sparked a deep rebellion in him against destiny, fate, the gods, and the world's entire order. A reaction that is truly understandable given his circumstances. His segments were him, more or less so Omega was the segment who, out of all the segments, was the version of Zandik most bitter at the world. Selfish, egotistical, he embodied Zandik's absolute worst traits. That is why he wanted to stop and destroy the way the system of fate operates in the world (Irminsul etc). His actions made sense - after all, what could a newborn baby possibly do to deserve the cursed fate of a heretic? Why does destiny randomly select people to bear one fate over another?
Like, this is beyond sad and tragic. And I understand why people hate on Dottore, but he's not a villain who's born evil or a type of "I do evil things because I'm evil hehe!" villain. He was doomed since the birth because the fate of Teyvat is unfair and stupid. He wanted to change that, not only for himself. He was the one to finally see that the fate of this world blindly decides who is blessed and who is cursed. He actually... made the world better now. He corrupted the Irminsul and Nahida needed to burn it. He achieved his goal. But at what cost...