Sephiroth had a much more detailed backstory
Too bad it's entirely clichéd.

and killed off a main character.
So does Kefka indirectly if you don't save Shadow on the Floating Continent.

Ghestahl was the one who did most of the work. Kefka
Ghastla didn't poison Doma, resulting in the deaths of Cayenne's family. Ghastla didn't explicitly say that the deaths of hundreds of people at once gave him pleasure. Ghastla didn't kill off General Leo. Ghastla didn't mass-murder the espers at Thamasa. Ghastla didn't destroy towns at a whim with the Light of Judgement. Ghastla may have been the one who built up the empire, but he certainly wasn't the one responsible for the most villainous actions in the game.

He also didn't even really destroy the world; the Warring Triad did.
Er no, Kefka moving the statues is what destroyed the world. If Kefka hadn't moved them, they wouldn't have done a damn thing. So basically what blackmage said.

Anyone coulda moved a bunch statues.
But not anyone would've. Ghastla, who you just implied was the real villain in the game, explicitly told Kefka not to.

Heck, Shadow did it at the end of the Floating Continent.
Yes, to prevent the devastation from getting even worse than it would've been if they were more out of balance.

Not to mention he freaks out at the sight of his own blood. And he is surprised that there is sand on his boots when they're WALKING THROUGH THE FREAKING DESERT.
So? Those things are just comical. Not everything about a villain has to make him a badass.