I would presume he used it any time it was dark. Beyond that and torturing people, I don't see much use for it for a god (being that peace-loving Hephaestus occupied the smith job, and he only used it as a tool). I guess if Hermes was a god of a modern mythology, he'd be using rockets n' stuff. Hestia could be an option, but Goddess of the hearth doesn't compare to the inventor of fire.
Hades is much more synonymous with the underworld. When you think about Greek mythology, nobody used fire as their go-to power. The Greeks were more about getting high on Aether, the celestial bodies and the natural elements. If mountains, lava and fissures is what you want, you're going to have to look among the Titans. Those were more about the primordial state of things. Hermes as an FG would be fine without the Destroyers, but I don't see why he can't have them considering they're one of the best Fire cards. His deck is almost only fire regardless. I don't get the "lava" vibe from him at all.