Just a little angst.
He'd never admit to how much it sickened him when the knife plunged through flesh into his brother's grace. The way Gabriel's hand tightened on his arm and the golden eyes bored into his. He watched as the light faded from them and he felt his heart break. Gabriel, who was so carefree, so filled with laughter. Gabriel, who had just wanted his brothers to stop fighting.
He let the body fall to the ground, the wings scorched into the floor. Rage and sorrow warred within him. Sorrow over the death of his brother, sorrow over the fact that he had been the one to wield the blade, that he'd had no choice but to kill him. Rage at the ones who had caused it. Rage at the Winchesters. Without them Gabriel would've stayed out of the fighting and he wouldn't have been killed.
He knelt next to his brother's body and his fingers ghosted over the eyelids, closing those golden eyes one last time.
"I'm sorry," he murmured. "I'm so sorry."
Lucifer leaned forward and pressed a kiss to Gabriel's forehead before he was gone, leaving nothing but destruction in his wake.