He wasn't a child. That is an interpretation some fans have owing to his form following SoS, but when you compare how he is referred to across languages from his short story, the overall meaning is more consistent with an age between late teens to young adult. I do think Zodiark inherited his persona and desire to save the world and therefore has that impulse of heroism in him, but again the thing to consider here is her stated intention, as per the cutscene in Anamnesis Anyder, is to check his power (presumably as a means to cease the return of ancients in Zodiark and let the "new life" - whatever it was - inherit the world.) Not necessarily to split up the world into a bunch of shards. Then you have this from Emet-Selch in the French version:
Emet-Selch : En tant que garde-fou de Zordiarche, Hydaelyn était dotée de capacités tout bonnement extraordinaires.
= “As Zodiark’s safeguard, Hydaelyn was endowed with simply extraordinary capabilities.”
Emet-Selch : Des attaques qui, en plus de toucher l'enveloppe matérielle de l'adversaire, tranchaient jusqu'à son essence même... C'était du jamais vu.
= “Attacks that, in addition to touching the material envelope (body) of the opponent, cut to its very essence… It was never seen before.”
"Never seen before" in this context can easily mean "not fully understood". Although she and her posse claimed Zodiark was not a solution, a possibility to consider is that this entire conflict wedged between the groups was instigated by another party, feeding off concerns, justified or not, about Zodiarks' power. Telophoroi as a term certainly sounds like something rooted in the ancient world. It's easy to conceive of the odd immortal life form wishing to end its existence and, with it, the Lifestream/Underworld that would return its soul in due course to the living... possibly trucking with powers beyond the ken of the ancients to achieve this.
If Zodiark stayed the original incident, it stands to reason that he could prevent it again, and so needed to be removed. We don't know if Venat intended the Sundering, or thought the power would just weaken Zodiark enough for him to not outright destroy her as they established a co-existence. Either way, on this basis a possible outcome by whichever third party instigated the conflict is that it'd destroy both Zodiark and the world with it... but instead, it sundered them both. The only wrinkle here is Elidibus's claim at 5.0's epilogue that she and her summoners both intended for this truth of the world not to be known to the sundered life forms, which does suggest the sundering was intended, but on the other hand their suggestion to the Convocation may have been to phase out the ancients and he may be interpreting it in that light. Admittedly it's all (varyingly wild) guessing on my part too, but it would not be the first war in the setting started through a misunderstanding instigated by a third party, that dragged on and on...with Azem distanced from both Primals (we can debate as to why they ignored her group's solicitations, but it could indicate she was not seen as much of an improvement over the situation either...), and Hades evidently "out there" in some sense, I think they will be the answer to the 5.0 epilogue question, of who will end this war. After all, Venat never maligned the Convocation's intentions but stated they too only wished for what was best for the star. Given how Zodiark, having delivered on abating the Final Days, had another primal sicced onto him after Elidibus had departed to go mediate the dispute, I imagine there is much residual rage and confusion in him.