Zemus did infact Die, Zeroumus is the incarnation of Hatred. You can't kill hatred. Zeromus appeared more as a form of Metaphor, saying as long as hatred exists, there will be another like me, or something to that effect.

And that is not a good enough reason Crystal. Thats stretching the meaning of words. Plus, how can Garland Create something Eternal? Also, why would he make it so powerful if it was only meant to sort the souls? It is a spiritual entity, so your party couldn't fight it under normal conditions, so making it powerful is completely pointless. Also, Kuja kills the party, if he isn't so omnipotent and was destroyed, how did he bring them back to life? Garland obviously doesn't have methods of doing that, otherwise he could have found a way to use NEcron to bring himself back, which he couldn't.

Also, the chose of the World eternal. If SoS was right, then why didn't just say he was immortal? Or everlasting? OR something else like that? It is doughtful that square would put in such a misleading phrase if they meant NEcron to be the Iifa tree. Also, FF has a strange habit of giving you an explanation for anything important by the end. It explains whats going on, the reasons behind everything and their motives, yet there is absolutely nothing but speculation from a fan to connect this. Two speaches shared many similaraties, so what? IT doesn't mean that they were at all connected. It sounds more likely that if Necron were able to watch Kuja, he could also watch Garland. Garlands speach seemed to echo the disire for humans to live withough fear and all that, and NEcron, not being human and obviously not fully understanding human though, he echoed Garlands speach to describe his goal.

HE didn't kill the party because that wasn't his goal. Lets say he's garlands creation and he actually intended to kill the party. He has control over souls and sorting them, he should have been able to easily eradicate the party with this ability, as they were nothing but souls. So even if he isn't all powerful, his power over souls could have crushed them. However, if his intent wasn't to kill them, but to test their will to live, as seems to be the generally excepted view, then Necron being the Iifa tree makes not sense. Either way, Eternal Being or Iifa tree, it would have easily destroyed the party, meaning it is far more logical that Eternal Being, is right.