Katherine was going to make him choose. She had led him to the brink, and left him alone with probably the only two people in the world who would choose him. But he could not return the favor. Not to both of them anyway. Katherine knew what she was doing, and she knew that making him choose between his brother whom he had sworn eternal torment against, and the one who looked so like the Katherine he had once loved, but possessed the qualities that allowed her to really care for him, something he desperately needed to exist; but she was not his, her heart longed for Stephan, and Katherine knew that making Damon choose between Stephan and his lover would drive him mad. It was what she wanted, and, if history was any indicator at all, Katherine always got what she wanted.
Damon had already accepted his feelings for Elena. If he had been any other respectable man, he might have tried to deny them out of respect for his brother, but, as Damon had no use for such integrity, it was easy for him to realize that he loved Elena, just a bit difficult for him to watch her open the door to his house, come into his home smelling so delicious, and then for him to try to drown out the sound of her love with Stephan with large quantities of alcohol and sorority girls.
But Katherine clearly did not care about the war raging in Damon's head, because it seemed like she would get her wish no matter who Damon chose. It seemed that if she could not ensnare Stephan's heart, then no one could. And it was obvious how much she valued Elena's life, so this knowledge that Damon had concerning Katherine's thought process weighed heavily in his decision. He knew that it was a very real possibility that no matter who he picked, Katherine might just kill them as well, and then Damon would simply lose everyone he held dear. His whole world rested on this one decision, and he'd be damned if he was going to make up his mind in a frantic rush of anxious emotions which threatened to overwhelm him at any second. One minute from now, he might collapse under the pressure and resort to the part of himself he usually had on showcase for the rest of the world to see, the part that acted like nothing mattered and he cared for no one but himself, just lived in the moment and loved every decade of it.
"I choose Stephan." Elena and Stephan both stared at Damon. Their shocked expressions showed that neither of them expected those words to leave Damon's lips.
Katherine snickered. "Wow, Damon, bravo. I thought you were such a bottom-feeder that you might not even choose family. But… I can't really let my doppleganger die, so your choice is irrelevant. Tsk, tsk." And with that, Katherine staked Stephan's vervained body right in the heart. Elena stared and made no move to wipe at the tears running down her face, and she stayed like that until Stephan was dead. Katherine let her. But Damon saw an opening and tried to lunge at Katherine, the vervain in his blood slowing him down just enough for Katherine to block and doge his attack. Now a few feet to his left, she laughed at him. Or maybe she had just predicted his attack and the vervain was affecting him more than he thought. As Elena started to audibly weep, Katherine made shushing sounds and whispered as she stroked Elena's hair and pushed it out of her face. "There there now, I made him, so I get to destroy him. It's only fair, isn't it, Elena? I'm sure darling Damon over there is –" Katherine faltered as her eyes flicked over to the decaying wallpaper that Damon had just been kneeling in front of. She flashed over to where he was hunched over his brother's body, the wooden stake flung away onto the concrete floor. Stephan's blood was everywhere, and he wasn't healing. Damon didn't even feel his head hit the solid floor when Katherine pushed him off her most recent victim. He didn't even register when Elena took advantage of Damon drawing their captor's attention to plunge a syringe with a clear liquid inside into the carotid artery of Katherine's neck.
When Katherine was a heap on the floor, Elena sat down beside Damon and cradled Stephan's head in her lap.
"He's dead, you know." Damon Salvatore, smooth as always.