You kind of missed my point there... I was trying to discern the source of Zenos' overwhelming power, not necessarily comment on his mental state. I would nonetheless argue that anyone who considers everyone weaker than him mere fodder for killing as he wishes, and thinks anyone who doesn't embrace violence for its own sake is less than human, is not of entirely sound mind. I admit that is just my opinion, though I don't think Eorzea (and going by his reputation even Garlemald) really disagrees.
Anyway... nethicite is a special magic stone from Ivalice. It functions by absorbing magical energy (Mist / aether), then releasing it for power, and comes in two flavors: deifacted and manufacted (god made and man made). Deifacted nethicite functions mostly as a nuclear weapon and takes a long time to charge. Manufacted nethicite has a wider array of uses and charges faster.
The specific use I'm looking at is its ability to augment the power of a fighter. One of the five Judge Magisters, Bergan, had implanted manufacted nethicite into his body to raise his power - but it eroded his sanity to a degree as well. I can't help but wonder if Zenos did not do something similar, given how he's able to shatter Y'shtola's thus-unbreakable barrier with one hand and apparently kick our ass even though we've taken on Regula van Hydrus, one of his peers. (That one nobody can argue - we had no Blessing of Light active and canonically defeated van Hydrus in single combat.) Granted [manufacted] nethicite is not known to be extant on Hydaelyn, but I admit I am only hypothesizing.
I'm not saying we should be the greatest fighter on Hydaelyn... but given what we've fought, there must be something behind Zenos' power that lets him toss us around as he does.
(See, Theodric? I'm actually pretty knowledgeable on Ivalice. I just don't like the already-myriad references being compounded with a name drop for the 4.1 24-man.)