@Owl @Jens_Rasmussen WE TALKIN SOULS LORE? HOO BOY!

Mankind certainly does not equal nature, at least not in the setting of Dark Souls. The world as it is know is established by the gods, Gwyn and his kin. When you look at the abyss and Oolacile it seems that the Dark soul of humanity is a force strictly incompatible with that natural order, corrupting and destroying.

About the Undead curse, we don't know that ending the Age of Fire removes the curse (it's certainly still present in Dark Souls 2 and 3), but there is a lot of mystery to what ending the Age of Fire actually involves. It might though, the Dark Sign seems to be made of fire and was Gwyn's Final Solution to the AQ (Abyss Question). But it might not make a difference if you link the fire or become the lord of darkness in that one respect.