Save Yourself by My Darkest Days
Stein knows what he is doing is wrong.
It is there in the back of his head every time he looks at his partner, whispering to him every time he lets his Madness take hold, purring self-loathing when he sees Spirit's face collapse into tears on his behalf. He is taking advantage of the weapon, letting the other boy care about him, care for him, when he knows in his heart that he is a broken thing that can never be put back together, that was never whole in the first place. That he cares about Spirit in return, that he aches for the comfort the older boy provides, that the weapon's voice can curb the wail of his insanity to a faroff whimper: these are incidental and accidental, nothing that Stein can control and nothing that should hold Spirit where he is. The meister knows entirely that something is wrong with his perception of the world, that other people do not casually disregard the feelings of those around them, that there are such things as morals and that others are restrained by them. Spirit is a better example of this than anyone else; Stein has never met anyone else so burdened by caring, for strangers, for friends, for his family. Above all, Spirit is dragged down by the infinite difficulty of caring for Stein himself.
If he were a better person, Stein would tell him to leave. He would push the other boy away, tell him to find a better partner, maybe go over his head and have him officially reassigned. He has always been distant from those around him; all it would take would be the act of relegating Spirit into the category of "other" and it would be done. But he does not know if he can remove the weapon from the inner sanctum of his self now that the older boy is there, and more terrifyingly he does not know if he would be able to make himself pull away if he knew it were possible. If he looks too closely he may find that he has always been able to, that it is a matter of unwillingness, and if he finds that fact Stein is horribly afraid that the unfamiliar sting of guilt and love will control his body as thoroughly as his Madness ever has and do the separation for him, and that he cannot bear.
Not looking doesn't stop him from knowing, though. When the Madness comes on him, jerking him awake with the echoes of his skull, he knows. He knows when he drags himself down the hallway to Spirit's room, knows when he pounds on the door until the weapon appears bleary with sleep and panicked with concern. When Spirit pulls him to lie against the other boy in the weapon's bed, when Spirit wraps his arms around Stein's too-thin shoulders, when his fingers are stroking the back of the meister's neck and tracing warm patterns on his scalp, Stein knows. When the screaming in his head has faded and he can hear the tears in Spirit's wordless murmurs and can feel the damp of concern against the weapon's cheeks, he knows.
The Madness goes quiet at times like that. Stein shuts eyes gone inexplicably heavy with moisture and winds his own arms around his partner, and his fingers clench into fists in the back of Spirit's shirt while he tries to tell the weapon to leave, that this is only a treatment and not a cure, that he will never be fixed and this will only get worse and there is nothing at all for the other boy but the constant losing struggle to make Stein into a functional human. He tries to tell Spirit that the weapon doesn't understand, can't ever understand, and that Stein will just keep taking from him, bleeding off his endless affection, shadowing the perfection that is his partner. But the words always stick in his throat, choking him on his inability to protect his weapon from himself, and the only thing he can do is dig his fingernails into the nerves of his palms while his tears condense on Spirit's ever-forgiving skin.