The Zoldyck Family treated the matter as a grave threat. Even Great-Grandfather Jagg, who rarely showed himself, had specifically called Alluka to his room and kept her there for half an hour.
According to Alluka, when she later told Killua about it, Great-Grandfather Jagg had simply stared at her for a long time, asked a few questions about her daily life, and then let her go.
If even Great-Grandfather Jagg had reacted like that, the others had been even more intense—especially Killua and Alluka's mother, Kikyo, and his older brother, Illumi.
After that, Nanika began appearing frequently.
Kikyo and Illumi used the lives of the Zoldyck Family's butlers and servants to conduct several controlled experiments, testing the various conditions attached to Alluka and Nanika's abilities.
"Wishes," "requests," "chain deaths," and so on were all discovered during this period.
If Killua hadn't heard Kikyo and Illumi repeatedly warning each other, he wouldn't have known that Alluka and Nanika's abilities came with so many restrictions and such terrifying consequences.
Alluka and Nanika had never demanded that he do anything so outrageous when they made requests of him.
Only later did he realize that these conditions and costs didn't apply to him, which was why he had never noticed all those dangerous rules.
Alluka and Nanika would never hurt him. That was the essential precondition for activating any of their abilities, and their highest priority.
By this point, Alluka's movements and contact with other people had already been restricted. None of it was her fault—the ones at fault were the people who made wishes.
If no one made a wish in the first place, there would never be a request made of the next person. Why couldn't everyone understand that?
At the time, Killua was deeply unhappy with the way Alluka and Nanika