I tend to treat soulstones kinda like how the Zodiac Stones are in Final Fantasy Tactics (minus the whole transforming into eldritch abominations thing...). The opening attract screen of FFT vaguely describes that the zodiac stones in that game are stones that have the memories and power of past heroes (and villains) "etched" into them. As a result, anyone who forms a pact with a stone basically could draw on its power for good or for evil. The wrong person using a stone would draw out its raw power triggering a transformation in those people externally into the form of the Lucavi (see: Wiegraf turning into Velius/Belias). On the other hand a person with strong enough good intentions could draw out the good side of the stones, such as when Rafa's brother Malak was nearly killed and the stone saved him at Rafa's plea to save her brother.
This may seem like a far cry from soulstones in FFXIV but it works on the same principles in some of the stories though it's more of a case of the soulstones/powers of the soulstones overwhelming NPCs rather than NPCs being inherently evil in some cases. The whole idea of White Magic being abused in the 5th Astral Era and the resulting Umbral era is a good example of the power being used for wrong and the Warrior questline and trying to control the Inner Beast is a good example of the power overwhelming a user. The instruction/memories on using the power is there, but the effects on a person can depend on how they decide to use it.
This may seem like a far cry from soulstones in FFXIV but it works on the same principles in some of the stories though it's more of a case of the soulstones/powers of the soulstones overwhelming NPCs rather than NPCs being inherently evil in some cases. The whole idea of White Magic being abused in the 5th Astral Era and the resulting Umbral era is a good example of the power being used for wrong and the Warrior questline and trying to control the Inner Beast is a good example of the power overwhelming a user. The instruction/memories on using the power is there, but the effects on a person can depend on how they decide to use it.
