The Master of Death

It is not an orignal story. Orignal is available on Ao3 by Quine.
"A second chance," Death said.

Harry stopped in front of a bench and turned to look at the being. "A second chance at what?"

"Life."

Harry laughed bitterly. "Because that has worked out so great the first time?"

Death tilted its head. "You've changed," it stated.

"Life does that to you," Harry said and his mind felt clearer than ever, "Or should I say the last seven years. I never really left this place, have I? I lived a life feeling not quite dead and not alive either," he spat.

"I know how you feel," Death said after a moment, grinning.

"Oh, do you," Harry retorted and pierced Death with a look.

The being's smirk widened as it leaned closer.
"Bored."

Years after the war, Harry Potter lives an apathetic life. When Death offers him a second chance he takes it.
Accompanied by the being, Harry travels back to the summer before Sirius dies. Inside his younger body and the memories of an older self, Harry realizes that being connected to Death may have twisted his morals a little more than expected.
Seeking to make up for the boredom of past years, Harry isn't above carving his own space between two sides to even out the playing field.