Grimmauld is destroying Sirius. It's a plain and simple fact that stares Harry blatantly in the face every time he peers into his godfather's eyes and sees nothing but phantom pain, nothing but nightmares and broken hopes and the tainted glint that's the only remainder of an empty Firewhisky bottle sitting innocuously on the corner of the dining room table. Harry stares at it, as Sirius is doing, but instead of seeing a reprieve and the freedom that Sirius so desperately desires, Harry sees hatred and hurt and the few remaining drops of the poison that's taking Sirius away from him.
Remus tries to tell him that it's not worth it, that Sirius' rages aren't going to stop because it's Harry trying to take the bottle from him; Sirius has chosen his path and this war isn't leaving them the time to do anything but fight. But Harry's had enough of fighting and can't help but watch in desperation as Sirius pours himself another drink, hours after Harry had tugged him up into the bedroom, tucking the man in and ignoring the stench of liquor with absolute grace.
He can't let this happen, can't let Sirius die, can't let Grimmauld and Firewhisky whisk Sirius into some world where there were never any Dementors, or war, or betrayal and lies. Where James and Lily Potter are still alive and Azkaban was nothing but a wisp of fear in the back of everyone's mind. Sirius isn't good at facing pain, Harry knows this, but he won't be alone and Harry tries telling Sirius this as he wrenches the bottle of alcohol from trembling hands that have more strength than they should.
It doesn't do any good; Harry finds himself cast to the floor with angry gray eyes flashing somewhere above him, and he realizes, belatedly, that they're alone in the house. There's rage and pain and drunken focus on Sirius' features, and Harry knows them that if he tries anything, that Sirius will lash at him like the wounded animal he is. It doesn't matter that he's Harry Potter, that he's the savior of the Wizarding world, that he's only seventeen and has faced more horrors than some people twice his age, that he's Sirius' godson. None of that helps Sirius in any way, and Harry finds that he prefers it this way.
Sirius winces and turns away from him, long, tangled hair snapping with the force of the movement; Harry watches as a shaking hand passes over the blotchy surface of the table and doesn't move or breathe, much less allow himself to think until Sirius is out of the room, cradling the Firewhisky and humming a soft, haunting tune under his breath that Harry hears in his nightmares later that night.
Harry finds the man sprawled in the hall beside his parents' bedroom door, eyes glassy and wide and a broken half smile crosses his features when Harry crouches to his level; the young man wonders what he has to do to get through to the older, and barely reacts when a soft, liquor flavored kiss his pressed to his mouth briefly. Sirius rests his head on Harry's collarbone after he pulls away and whispers,
"I don't need your help, Harry."
Harry's surprised at just how sober Sirius sounds, and brushes his own kiss along the man's jaw as he runs a gentle hand through greasy hair. He knows Sirius isn't going to remember this later, but nods nevertheless and whispers back, "I know. But it's all I have to give."