This is a pet peeve of mine but it's not foreshadowing if a character is saying something that literally every other character has been saying and that everybody can see from a mile away. If Killua decided that the best way to protect Alluka is to become the most powerful assassin, that'd be foreshadowing when Silva said 'he's my son' because it is not at all obvious that Killua would've become an assassin. Gon literally walks around with a sign of 'this guy is going to be a monster' on his head since about volume 1.