Years ago when I first read the few details about Geb, and there were scarcely any at the time, my head cannon started filling in things. As a fellow lover of the Souls series, I imagined Geb and it's conflict with Nex took a more noble-tragic route.
At the time, I recall that it was a plague that struck Geb's populace (or so I thought) and was slowly killing them off. In my head, Geb wasn't so malicious and in his attempts to save his people he turned to necromancy. At first, he went to completely-made-up order of knights and paladins, asking them to give up their lives to become more powerful. All of them agreed without questioned, and through a ritual became an order of Deathknights, called the Entombed. Spirits bound to their armor and weapons. But even that wasn't enough to save his people, and he proposed the same to his dying people. Out of anger, desperation, and all manner of terrible emotions most agreed. And so it went on, knowing that while he was prolonging his people's lives as it were, he was damning their afterlife to an extent.
With such a force at his command, one of his last acts as a living leader had such a battle that it ended the war with the death of Nex. And in his grief, Geb then killed himself and rose unintentionally as a spirit, now dubbed the Mourning King. Today, Nex is a neutral city much as it is in canon, but has developed a culture surrounding paying homage to Undead (in whatever form) ancestors, who linger and protect and teach their living ancestors. To a point that it is often a great honor to become Undead and live on.
There are principles though in place concerning mindless undead, and being as at peace with the living as much as possible, and not tolerating malicious undead. This still puts them at great odds with most nations and beliefs though, and that by nature they are Evil aligned, but not so much in deed if that makes sense.
