If he's the antagonist the reason should appear immoral to the readers, as a way to strengthen the impression of being the bad guy. Since he's the leader of a cult with magical powers, the obvious reason is to impress his followers with his unmatchable power, and therefore he should accomplish this all by himself via the application of his powers. This will make him look badass to the followers, and the readers, and therefore all the more intimidating as an antagonist to the hero.
Another practical reason to flat out destroy a planet would be to harvest the rocks and minerals en-masse or to extract the planet's core as a long-lasting energy source without wasting a second fighting the planet's military, especially if the faction had a Death Star Superlaser-esque superweapon or a spell to blow up a whole planet.
Edited by Mhazard on Apr 8th 2020 at 4:40:38 PM
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I like that idea. Maybe it's to add land to their planet.
My antagonist is a group who plan to destroy the planet by shattering a moon and letting the resulting asteroids destroy everything. But they need a reason for this, and I'm having trouble coming up with one. For context, the group is cult-like, and led by someone who possesses magic. They live on a planet with strong matriarchies and frequent fighting between countries.