On the rpg, at the first pages, I explain that. I discarded the idea of 'dying and reviving' that exists in Dark Souls. I imagine that this exists mainly for the game have an excuse for the player to 'resurrect' every time he dies. The other npcs, when killed, don't come back, even those that are undead too. Maybe they don't have the dark sing?
So, you could say that the dark sign is needed for revive? Or is the darksing just a mark done afterwards you became undead? Many things are left unanswered in DS. In the rpg, I just say that you can burn humanity to heal and, if you lose all humanity (5+ Karma), you became Hollow. If you receive enough damage (5+), you die.
That was a design choice. But you can easily make so that, when you die, you lose a permanent point of Humanity, so you would become hollow with Karma 4. That way, you would make the idea of slowly becoming hollow after each resurrection. I don't like that idea, preferring that if you die, you die.