Empire of Silence By Christopher Ruocchio

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.
On the wrong planet, at the right time, for the best reasons, Hadrian Marlowe started down a path that could only end in fire. The galaxy remembers him as a man who burned every last alien Cielcin from the sky. They remember him as a 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.
Fleeing his father and a future as a torturer, Hadrian finds himself stranded on a strange, backwater world. Forced to fight as a gladiator and into the intrigues of a foreign planetary court, he will find himself fighting a war he did not start, for an Empire he does not love, against an enemy he will never understand.