Watching this arc really is the agony and the ecstasy...
we've already seen it - and in that sense Knuckle and Shoot are well-matched opponents for him to the extent that anyone could be, given his terrifying pure power.
Missile Man" Nen ability. Welfin is wonderful at "sniffing out" treachery and danger, whip-smart - but he's also too calculating by half and sees threats where there are none because if it were him, he'd always be planning some sort of treachery so why wouldn't everyone else? This hyper-sensitivity and naked ambition causes him to hesitate where action is called for. He's incredibly dangerous, for all the reasons Meleoron says, but also his own worst enemy. I'm suspicious that he'd really consider going over to the other side as suggested in this episode, but I do think the notion gives him yet another reason to be hesitant in his actions.
Meleoron because he cannot stop to help Shoot when it might endanger the overall goal. Ikalgo dedicates himself to finding Palm because although he's never met her, she's a friend - because she's Killua and Gon's friend, and because she risked her life to try and defeat the King. For all the loyalty of the Royal Guard to the King, the Chimera are a bit of a mess - each caught up in their own world, and at the level below the R.G. a tangle of confused loyalties. And even the Royal Guard themselves are basically fighting independently at the moment, to some extent dancing to the Hunters' tune. It's ironic that humans - and ant turncoats - should be more loyal and acting with more cohesion than ants. But so it seems to be.
What did Zeno see in Killua's eyes that told him, "He's changed"? He certainly has - his loyalty to Gon, and his transcendence of the chains Illumi placed on him (though the psychological effects are clearly still being felt). Zeno's job is indeed done here - his role was to create the chaos that separated the King from his Royal Guard, and to give Netero his single combat at a remote location with few innocents in harm's way. He's been paid, and he (apparently) leaves - but clearly, seeing Killua was an emotionally significant moment for the old man. Would some part of Zeno, who lives by the mercenary code of the professional killer for hire, want to stay and help his grandson in his own struggle, which is not for financial reward but for the sake of those he cares about? I suspect the answer is yes.
Facing off against the terrifying Pitou is bad enough, but it's now obvious that Togashi has also set this up as a battle against himself for Gon (likewise, a frightening opponent). The purity of his own feelings, his seeming incorruptibility, may be working against him here, and the rage over the injustice of what the King is planning and what Pitou has done to Kaitou transforming into hatred. For most of the series we've seen Killua battle against himself as Gon's unimpeachable solidity acted like True North on a compass, guiding his friend whenever he was lost. Now, it seems, we may at long last be seeing the time come for that debt to be repaid - for Killua to provide the anchor to keep his friend and brother from drifting off course, the hand to grasp and pull him back from the darkness. It should be a beautiful and terrible thing to watch play out, and I suspect that drama is going to begin in earnest next week.