I know it isn't literally cancer hence why I wrote "like some".
also I don't think it's clear whether it's actually supposed to be an intelligent being or just some "natural" phenomenon.
anyway, I think you are missing the point that it basically also merges with the mind/personalities of the people that enter. all the people in the group had nothing left to live for / were dying already, so "the zone" might just created an environment based on their wishes. isn't it that they wake up right after entering and days have past and they have no idea what happened?
also the movies is from the perspective of Portman but the ending is that she might be a copy too, so we don't know what's even true. like maybe that was all part of some plan or something