He closed his eyes for a moment, muttering to himself. Opening his eyes, he spared a look at Crowley, as he uncapped the vial. He set the cork aside, carefully, so that it was out of the way, so if he was unable to dispose of it later, Crowley could call someone to take care of it without touching it himself.
The corners of his mouth twitched. "Whatever happens, dearest, know that I have loved you since the first I saw you, in the Beginning." Without giving Crowley a chance to reply, he brought the vial to his lips and swallowed the contents quickly.
The burning began instantly, tearing him apart from the inside. He hit his knees, then fell forward, face down on the floor. The heat seemed to consume him, and he could not fight back the screams. The glass of the windows shattered from the sound, the agony of the demon dying within him.
if Crowley had the presence to look closely, he would see the grey of the angel's wings turning to white, slowly, making the flight of the demon into Hell, as Aziraphale writhed in pure agony on the floor.