Starsifter

Geto finds himself forced to physically touch, to eat, the worst of what humanity has to offer. Curses. At the beginning of his arc, he does this out of obligation. He sees it as his duty, to protect the weak. The humans. Jujutsu sorcerers are taught that they are responsible for protecting the weak, regardless of the cost to themselves. Much like soldiers.

As he grows to resent humanity, as he sees more and more of what the supposed weakest can do to his fellow sorcerers, whether through direct action or the passive creation of curses, he finds himself with a dilemma. At his core, he is kind, he is compassionate. But, to be kind and compassionate to the "weakest" he cannot be kind and compassionate to the "strongest". To his fellow sorcerers/soldiers. Because their weakness is, to him, what is killing the people he cares about.

So, when faced with his final straw a village of people torturing young sorcerer girls because of ignorance, because of weakness; he makes the choice to distance himself from humanity in the most extreme way possible. By killing the entire village. By killing his non-sorcerer parents.

At the end of his arc, his sense of duty and obligation no longer applies to the weakest. It only applies to his fellow sorcerers. Whom he sees as family. Whom he does not hate, regardless of how their goals may differ. (Gojo even trusts that while Geto may hurt his fellow sorcerers to achieve his goals, he wouldn't kill young sorcerers unnecessarily.) It is because of his compassion for his fellow sorcerers, that he cannot distance himself entirely from the weakest in terms of proximity.

So instead, he places distance in terms of his attitudes toward humanity. This doesn't mean he's incapable of being polite or kind to people, he's a charismatic villain. He's polite to get what he wants, whether that be curses or money, etc. But he still calls them monkeys, still sprays himself with disinfectant whenever he interacts with them, still has no true empathy when it comes to their suffering.

Still kills them. His proximity to them is under his control, for the most part. He refuses to help them unless it benefits him and his goals. And when they outlive their usefulness, he kills them.