Mathew knows what he has to do. He reaches to the side, and his fingers find a long sharp piece of glass stuck in his fence. Something greater than coincidence has brought this weapon to him. Fate has come calling for Mathew, and laid out his path clearly. Mathew is ready to answer.
Arthur sees the flash of the makeshift blade before Alfred does, and subconsciously tightens his arms to hold his brother back and keep him safe. Arthur still doesn't understand.
Mathew will never kill Alfred.
Alfred sees the blade of glass too, and his eyes widen in horror. He lunges against his brother's arms. "Mattie no!" There is something beyond terror in his tone, and Mathew knows that if Arthur wasn't holding Alfred back, Alfred would take his life to save Matthew's. Mathew can't let that happen.
"I love you, Al."
His words are met by a heartbroken scream, and the glass flashes against his neck.
Then red.
Arthur's arms go weak, and Alfred is running, falling, hands reaching to Matthew's side, pulling him close. The glass clatters to his side, like a red stained-glass. And Alfred is crying, pleading, begging the universe to give him back, to grant Alfred this one boon.
But the universe is too quick to take Mathew, glad to put things right again. There are no final words, no choked gasp, no light in the eyes beind the cracked glasses. As quickly as his first meeting with Alfred had started, Mathew is gone.
Arthur is in a state of shock, finally understanding. He doesn't have time to process what's happening, their's a tone in Alfred's wail that terrifies him, hurts him, and Arthur remembers he's only an older brother, and his baby brother is crying in front of him. He doesn't have time to feel guilt. He needs to act.
Alfred doesn't feel Arthur's arms, awkwardly holding him, but he hears the words Arthur murmurs into his back, the only solace Arthur can offer. "He did it for you." Arthur knows his words bring no comfort, but says them anyways. To Arthur, this is the only condolence he can offer, the only acceptable reason for the tragedy. There's a shocking feel of calm in the universe, like some great wrong had been corrected.
To Alfred, his universe has never felt less right, and there is nothing worth the price he paid.