You were a demon devoted to Lucifer's return. You conspired with Lilith to form a plan to free him. She broke the seals while you dealt with the special child ─ Sam.
You manipulated him, tricked him. You lied and promised him things that you'd never do. You simply told him what he wanted to hear, and helped him through his grief. It was honestly too easy for you. Even after you revealed yourself as a demon to him, you managed to convince him that you were good. Poor Sammy didn't know how much of a lie that was. You got him addicted to demon blood, and told him it would make his powers stronger, only it didn't. His decisions were what determined that.
It was all to prepare him to break the final seal and become the perfect vessel for Lucifer. He fell for it all, and didn't realize until it was too late. He opened the door. Sam had fallen for your beautiful lies, and killed Lilith. He thought he stopped her, though her death was all part of your scheme. She had to die to free Lucifer, and to fulfill the prophecy.
As the realization slowly came to him, you went on and on about how great your plan was- how great you were. His heart was thudding in his chest as he glared at you. He didn't know what to feel. Heartbroken? Angry? Confused? Scared? Maybe it was all at once. All the while you were ecstatic.
“You bitch!” Sam shouted, his mix of emotions swirling into rage. His voice echoed throughout the chapel. “You lying bitch!” He tried to use his powers, only to fail to do so. He felt drained, weak from even trying to attack you.