The Black Materia

[Sephiroth]

I guess my issue here, is how does a planet loving race create a "planet killer" spell?

It was not created by the Cetra. It was a gift from Gaia itself. No Cetra would have been able to summon Meteor, otherwise Sephiroth wouldn't have had any need of the Materia itself and just absorbed it out of the lifestream like he did with the knowledge about the Materia itself.

Of course it was all for a story. Not everything in Final Fantasy VII is based on 100% perfect thoughts.

Thus, in the days of the Ancients, they didn't need Materia.

The Cetra were able to summon the planet's power without materia but it doesn't mean they haven't used the mental instruction manual of other Cetra. After all they didn't have the same knowledge. However there is no use to talk about that since it is not mentioned.