Empire of silence / Christopher Ruocchio.

Enhanced descriptions from Syndetics:

Hadrian Marlowe, a man revered as a hero and despised as a murderer, chronicles his tale in the galaxy-spanning debut of the Sun Eater series, merging the best of space opera and epic fantasy.

It was not his war.

The galaxy remembers him as a hero: the man who burned every last alien Cielcin from the sky. They remember him as a monster: the devil who destroyed a sun, casually annihilating four billion human lives--even the Emperor himself--against Imperial orders.

But Hadrian was not a hero. He was not a monster. He was not even a soldier.

On the wrong planet, at the right time, for the best reasons, Hadrian Marlowe starts down a path that can only end in fire. He flees his father and a future as a torturer only to be left stranded on a strange, backwater world.

Forced to fight as a gladiator and navigate the intrigues of a foreign planetary court, Hadrian must fight a war he did not start, for an Empire he does not love, against an enemy he will never understand.