He removed the needle midway through the arc, shouldn't he have remembered Alluka? He could've used her to just kill the threats in the arc and prevent any other deaths.
In theory, he could've use Nanika for anything he wants. But that would give no sense to the anime.
We can ask the same question for Saitama in One Punch Man : "Why didn't he kill Orochi directly, or Garoh, ..."
It is very important to establish consistency in time and geography. When Killua went to NGL, he didn't know anything about what he was going to deal with. He only paid attention to being with his friend Gon. NGL is far away from their home.
Second point, Killua would never recover from being weak and not able to protect Gon. He's a Hunter, he can't ask for help in such adventures.
Final point and most important, Killua hates the way that Nanika is seen as an object, and not for her human side. He would never use her for anything exceptional. Gon's case was unique, he was going to die and he knew it, he saw everything.