So, this was born from me watching clips of the episode where Jeremy kills Kol. I always hated how no regard was given for the unknown scores of vampires that were murdered that night, all for the cure. So, I though, what if one of those vampires wasn't so unknown? So yeah, Damon didn't kill Lexi in season 1 (my idea being that she through him into a wall when he tried) and she's part of Kol's sireline, though didn't know it.
The first bit is canon, though it veers off toward the middle.
Jeremy paced back and forth in the main foyer of the Salvatore home, staring at his mark, which was noticeably not growing, "It didn't work," He growled.
"It took time for Finn's line to die off," Bonnie said from the couch, attempting to sound sympathetic, despite her abhorrence for the plan, "It will work."
"…But what if it doesn't?"
"Where's that Gilbert optimism?" A voice said from the doorway.
Everyone turned to see Damon walking towards them. Elena smiled and walked up to him, hugging him as he muttered "Hey."
The front door creaked as Stefan walked in with the headstone, and an obviously irate Lexi stalked in behind him.
"Looks like I didn't miss much," Stefan muttered after a few moments of awkward silence, "Got the headstone," he put it on one of the end tables.
"We're just waiting for Jeremy's mark to grow," Elena said tensely, pointing to her little brother, who suddenly looked as if he wished he could be on the other side of the planet, "Klaus is locked in our living room."
"Temporarily," Bonnie interjected, "I, I uh, drew on the new moon to bind the spell. We've got three days to find that cure, four max."
"If we don't, we might as well look up Katherine Pierce and see if she wants some company in hiding, cause he will come after us." Jeremy muttered.
"We all are going to have to do that anyway," Lexi quipped in, glaring at the young hunter. Her annoyance with the situation was growing, especially as no one seemed to actually grasp the repercussion of tonight's actions.
"We'll find the cure," Elena said, oddly optimistic, "Now that we've got Rebekah taken care of, all we need is Shane."
"Yeah, about that," Lexi said, hoping this at least would job someone's senses
"What?" Elena asked
"I didn't need, she's on our side" Stefan said simply.
"On our side?" Elena asked incredulously, "Did you really just say that?"
"Yeah, she handed over the headstone," Stefan motioned to it, "And she wants to find this cure more than any of us."
"Why would you possibly think that you could trust her?" Elena attacked.
"Let me guess," Damon said before anyone else could get in, "She swore her allegiance to you while you were naked in the sack."
Everyone just stared at Stefan in shock, before Lexi interrupted.
"Okay, enough," She walked into the middle of the group, and looked to all of them, "I was hoping that you kids would come to your sense on your own, but between Elena's idiotic levels of optimism and you," She pointed to Stefan, "Thinking with the wrong head, apparently I am the only one to comprehend the reality of the situation."
"Yeah, and uh, what's that?" Damon asked, mockingly.
Lexi turned to face him and the group behind him, "Well, I'm not going to go into the morality, or lack thereof, of committing genocide because if any of you people could even comprehend that, you wouldn't have done it. So allow me to explain through the aspect that you care about most: yourselves. Number one, you people killed hundreds upon hundreds of thousands, if not millions, of vampires. Do you people actually think that none of them had any friends or allies outside of their sireline? There are going to be a horde of beings, vampires, witches, God knows what else, that are going to be furious, and going to find out what happened to their friends. And guess where that trail ends, hmm? Us! So no, Jer, it's not going to matter how long it takes us to find the cure, it's not going to matter if you can use it on Klaus or not. Hell is about to rain in on this parade regardless, and you!" Lexi turned to face Stefan,
"If you actually think that Rebekah is on our side after the lot of you murdered her brother, you're completely insane. The moment she gets the cure, she's going to slaughter everyone. Including me, just for knowing you. So, thanks for sending me on a stupid goose chase while you all screwed me over!"
"The moment she takes the cure, she's human. Then she'll be easy to deal with," Elena said, trying to calm Lexi down.
However, ever ounce of anger Lexi had been building over the past few months had just been given a release valve, and it was going to take a lot more than that to stop her.
She turned again to Elena, "Oh really? Just like that? So easy? Yeah, sweetheart, did you miss the part where it was revealed that Rebekah's mother was a witch? And uh, what do you think that makes her? What does that make all the Mikaelsons, sans vampirism?
There was dead silence in the room as everyone grasped what Lexi was saying.
"A witch," Bonnie said.
"A witch, one trained by one of the most powerful witches in history. The one to whom making Originals is like baking cookies. The one who took a crack in the Other Side, and turned it into a freeway. The one who nearly wiped out all vampires. Except Rebekah won't be consumed with guilt and sorrow, no, she's going to have rage. The moment she takes that cure, she will hex us all faster than we can say 'time to die.' Whether it's Mikaelson vampire, Mikaelson witch, or friends and family of the people you mass murdered for no reason other than to get the damned cure, your idiotic little plan Elena Gilbert, has signed all of our death warrants."
"Lay off her," Damon threatened, getting between Elena and Lexi.
"Or what, you'll smirk at me until I die? This is your fault too, you know. All your faults. If you people were willing to tell the damsel 'no' every now and again, she wouldn't think she could get away with genocide! When did this happen Stefan?" She turned to her friend, "When did all of you become so selfish that you'd sacrifice anything and anyone for your ambitions? What happened to the guy who just-just"
Lexi made a wet, heaving sound as blood spurt out of her mouth. At the same time, Jeremy began to scream as the Hunter's Mark began searing its way up his arm and chest, finally reaching completion.
"LEXI!" Stefan screamed as she collapse. He ran over and grabbed her. He looked at her, his oldest, and for a long time only, friend. She looked at him, the man who was her friend, with tears in her eyes.
"Was…it…worth it?" She gasped out as her skin turned ashen, veins beginning to protrude.
Stefan held her, her corpse, sobbing. Everyone else stood in absolute silence, afraid to say anything, and unsure of what they would say if they weren't.
Stefan slowly raised his head, still cradling Lexi's corpse. He look at Elena, tears streaming down his face.
"Was she worth it?"
Lexi, born Alexis Branson, born November 13th 1659, turned August 5th 1680 by a vampire named Jonathan. Jonathan was turned by a vampire name Kate, who was turned by…Kol Mikaelson.
I know that some of the stuff turned out not to be canon at a later time, but I thought it would work.