She whispered in his ear that she loved him. Tears dripped down her face and clung to her long, dark lashes. It physically hurt him to see her in such pain. He gripped her hands tightly, and stared into her eyes as the life slipped away from them. Her white dress was drenched with blood from where she was stabbed. He looked down at their hands, clasped so tightly together, covered in blood, and he knew that this was entirely his fault. For weeks now, he had been blaming everyone else, his mother, his father, even the witches. The guilt was overwhelming, and his own tears formed and his vision blurred. Staring down at her limp form, the pain of his betrayal forever etched on her face, it was more than he could bear, and he wished to die.
Eventually he stood, and glanced down at her face for the last time. Or so he thought. The curse was placed upon him by the witches and he hunted them down without mercy. He slaughtered them, their families, everyone. The screams and the blood did not lessen the terrible pain that raged within his aching heart. She was gone, nothing could bring her back and nothing could stop him from feeling the guilt, rage and agony.
Over time, his quest for vengeance was replaced by an obsession for breaking the curse. He and his brother, Elijah, searched for many years, inventing the sun and moon curse, hunting for the moonstone and the doppelgänger. The doppelgänger. The one with her face, the bottomless brown eyes he had stared into, the lips he had kissed for hours, the hair that he had tangled between his fingers and the soft skin that had quivered beneath his touch. He missed her so much, and to be with her, even for a moment, to apologize, to beg for her forgiveness might finally provide him with some of the peace that he longed for.
When he met Katerina, looking at her did not fill him with the feeling of happiness that looking at his beloved had, instead he just felt resentment that this girl had the privilege of looking exactly like her but was not actually her. She was a pale imitation in comparison, and he felt like he was being taunted. He refused to be around her, retreating into his world of alcohol and violence while the time to break the curse grew nearer. Elijah grew closer to the doppelgänger, which made him angry and fearful that he would try and prevent the curse from being broken. And while he knew that Katerina was not the girl he loved, he felt jealousy at her growing bond with his brother.
After she escaped, rage blinded him and he lashed out at Elijah, even though he knew, deep down, that it wasn't his fault. Elijah was blinded by love and he could relate. He travelled to Bulgaria, slaughtering Katerina's family in a fit of vengeance and then transformed his life into one of sex and violence. He could not escape the pain, he could not forget her, but he could provide himself with distractions.
For over 500 years he lived this way, until he learnt of the second doppelgänger. He was, of course, surprised. Killing Katerina's family was supposed to have prevented that. Even though his fixation on breaking the curse fuelled most of his time, seeing her face again was something that he wanted to avoid.
Upon meeting the new human doppelgänger, he realised that Elena was more like her than Katerina had been. And once again, Elijah had developed feelings for the doppelgänger. He felt sorry for Elijah, always falling for these impersonators because the original had always been out of his reach. He was surprised again and again by the loyalty and bravery that Elena showed, even in the face of her death. Just like his beloved, she was just trying to protect the people around her. But Elena was not her, and nothing would prevent him from breaking the curse. Not the rescue of the young blonde vampire and her werewolf friend, not the feeble attempts made by the youngest Salvatore brother to make him change his mind, nothing. This curse was going to be broken, no matter what.
She had died for this. He was going to break the curse and rain vengeance down upon the unjust world that had let an innocent girl be murdered. Nothing was fair; nothing would ever be the same for him again. And if he couldn't have her back, and he couldn't die, then he would make everyone feel just as miserable as he does.