Characters responsible for destroying entire planets/planet systems.

Unicron (Transformers) - He’s the planet eater, so it’s obvious.

Galactic (Marvel) - Similar reasons as Unicron.

Cyn/The Absolute Solver (Murder Drones) - Basically destroyed earth alongside V, J and N.

Grand Moff Wilhuff Tarkin (Star Wars) - Is the one responsible for destroying the planet of Alderaan by ordering the Death Star to destroy it.

General Armitage Hux (Star Wars) - Is the one responsible for the destruction of Hosnian Prime and the Hosnian System by ordering Starkville Base to destroy it