Gabriel told Sam and Castiel that he'd tricked Lucifer into killing the wrong guy.
Each had stood against the other, tried to reason for their side of the apocalyptic fight. It was for nought—a show for the three survivors who'd left the scene. The brothers had already come to an agreement. Gabriel didn't have it in him to kill his older brother, and Lucifer could hardly imagine finally battling Michael, let alone hurting the youngest of them who really hadn't wanted any fighting in the first place.
Lucifer hated to do it, though knowing the blade was a fake.