That's true. Though it's quite natural for Killua to make that mistake. In fact I wouldn't have been surprised if he had made the same mistake that Gon made. After all, the Zoldycks are assassins, that is, killers of humans. Killua is far better at fighting humans than anyone of his age should be due to his background, but when he lets his instincts and training guide him, it actually makes him weaker against opponents with nonhuman characteristics, especially those as ridiculous as these ant hybrids. I'm sure he would deal with dogs and such very efficiently (since assassins would need to deal with guard dogs), but how would one train himself to fight these things?