In what episode does Killua meet Netero who is getting ready to fight?
Are you referring to Episode 99? 

Just under 14 minutes into this episode, Killua is running through the forest and senses something, thinking to himself "I can't go this way." 

What he senses is explained later as the "murderous intent" released by Netero when he was about to unleash his Zero Hand technique on Meruem.