When Gon turns to speak to him, he certainly does see the blood - an arm fallen off, anther one being cut neatly and easily by Pitou, who has a rather playful expression on her face. His expression falls, breath hitching as he looks away.

"--If it's pointless, you should stay here. I'm not stopping until I find out who did this!" Gon shoulders are heaving. He can't look at Killua - not until the illusion has gone away.

Don't come near me anymore! I hate you!

Gon's voice could be heard in Killua's head, Illumi placing a number of needles into Gon's skull - again and again.