It isn't fair. She was Lucifer's First. First to free and twist and own and martyr. His grace still beats through her corrupted soul. Lucifer made her. Azazel crowned Sam, his own plot, but she is Queen, still.
For one moment, she wavered, and in that wavering, she threw a tantrum and tried to break her father's toy.
Did she not suffer? Will she not sacrifice all she is? She knows Sam will be glorious when Lucifer takes him, fills him. He will live, be loved, all for existing.
She thinks his head would look more beautiful on a spike.