The issue comes down to the fact Zenos has two readings as a character, both valid, and the writing recognizes this... and then totally denies your ability to actually get any meaningful resolution from one of the angles.

Firstly, Zenos could be a dark reflection of what it means to be a PC in an MMO. He really DOES get you, understand you are here for the challenge of it all, the desire to fight bosses and win cool fights. If that is how you relate to Zenos, his ending is totally fine: He is a mirror of you that understood key aspects of you but didn't get the whole picture, a 'Story Skipper' if you will, and his desire to duel you and your ability to accept him as an awesome fight allowed that resolution to work.

However, there is the 'reject' reading of Zenos... which also is the reading that works better with the actual themes of XIV, which can be summarized as 'people matter, regardless of who they are, and the ultimate evil in the world is to reduce other people's value or agency, be it by declaring them a member of a beast tribe or just saying they were your enemy born rotten.' XIV goes VERY far out of its way to show that people tend to become the way they are for a reason and that conflict usually is a result of pressures (such as from history in HW, or for survival in SB) that cause strife, and the best thing we can do when someone tries to hurt us is to understand them. Zenos, in this reading, is sort of the antithesis of the entire plot of XIV: He doesn't believe in any sort of meaning, actively uses other people for pleasure, discards those that bore him, and revels in pointless cruelty.

And that is, again, fine, Zenos invoked a lot of strong feelings in me as a player, but the problem is that there was no catharsis for those feelings. I could offer token rejection, but in the end my final dialogue choice with him, an attempt to tell him I really was having none of what he was offering, made him HAPPY and our final moments together were a contemplative one (With the implication he somehow burnt some of my essence, extra gross), where Zenos got everything he wanted from me and there was no real refutation of his unethical hedonistic nihilism. Suddenly the story forced 'Zenos was right about you' on you, and a villain who served well as an antithesis to what the story was about just got a weird bit of unearned reverence and closure both your character and you the player were totally denied.

Some really minor re-writes would fix this trivially (just... add a dialgoue option that makes Zenos frustrated and mad that he doesn't get what he wants from you in the end and attack you out of rage of not understanding why he can't get you to be his ultimate frenemy' to start the fight, and a rejection in the aftermath as well, you just really need a chance for Zeno's story to end with the anti-climax rather than yours), but they really fumbled it in the final interaction.