I don't know, IX had a villain who was created to cause wars and kill countless life on a undeveloped world so Garland could infuse the planet with the souls of his aging culture into their circle of reincarnation. The souls of the planet, unable to return to the cycle of reincarnation are turned into Mist which causes mutations in monsters and provokes the violent tendency of the planets inhabitants leading to more death.
Kuja then takes this Mist and uses it to create a race of beings whose only purpose is war and are expendable. He then causes the devastation of the two biggest world powers kills countless people and his home world. In the end, he decides to wipe existence out cause he can't bear the thought of life going on without him.
From a mature and philosophical standpoint, I would say that is pretty dark. Personally, the only "dark" thing Versus has shown us is the heavy use AC's color scheme (Black, dark blue, and sun bleach

) and we actually get to see blood. If you want a really "dark and mature" FF, play the original FFTactics.
