He is young — too young — when the war breaks out, but he's never really understood that until this moment.
He has learned, has understood, has known that the Death Eaters only follow a master because they are merely followers, unable to think for themselves; that they torture and murder those they consider subordinates because it makes them feel superior in their own minds; that they are nothing more than spineless cowards.
He can mock these people without remorse because they are weak, and because he knows he will never stoop to their level.
So why is his wand pointing unflinchingly at a man in dark robes and a bone mask, intent on taking a life?
Though his wand is sure, his heart is not, and the moment of uncertainty is enough for the smirking man's wand to turn to Fabian — oh Merlin, please no— who is lying on his back, clutching his bleeding side.
All he can hear is the blood rushing past his ears, pounding a steady rhythm, reminding him that he is alive, and if he doesn't act soon, his twin won't be, and just like that, all the hesitation is gone.
He will later vaguely recall chucking a small rock at the coward's head so that he won't have killed a man whose back was turned. But he can vividly remember the flash of green light, and suddenly his hearing is back.
There is a thud as the lifeless corpse of the Death Eater drops heavily to the cold, hard ground, gazing sightlessly into the clear, blue sky. There is a loud exhale that escapes his shocked lungs, momentarily stealing his ability to breathe. There is a pained moan, and the noise from his brother shocks him back into action.
He doesn't realize until later — after he has taken Fabian back home so Molly can fix him up; after he has paced outside the room his sister won't let him in for hours, snapping at anyone who dared cross his path; after he has been assured that his twin is still alive and kicking (quite literally) — that this makes him a killer. A murderer.
A monster.
And a bloody hypocrite.
"They say that what you mock
Will surely overtake you
And you become a monster
So the monster will not break you."