When it comes to the flexible and ever changing moralities of characters on The Vampire Diaries, it seems safe to say that Katherine is one of the most morally flexible of them all. She is a manipulative, terrifying femme fatale, but to be honest she doesn't do things just for the sake of being evil. Most of the time she does them just because she can.
Katherine is one of the few characters who seems to relish nearly everything that she does, and as a result she doesn't experience a lot of remorse or embarrassment over the awful things that she's done. And boy, has she done a lot. It's hard to say what is the worst of the worst, but these are the undoubtedly most shameless things that Katherine Pierce has ever done.
Killing Anyone For Anything
Katherine Pierce had a lot of ugly sides to her behavior, but what seems to be the worst thing about her is that she has absolutely no judiciousness when it comes to killing people. Even the worst villains in TVD history seemed to have occasions where they reconsidered killing people when they didn't really have to, but Katherine typically murders anyone for any reason that she sees fit in the moment.
Basically, if she's in the mood to off someone, she will. Other characters have a higher body count, but they at least seemed to have some rationale behind most of the murders they committed.
Hating Elena
Katherine definitely likes to be the star of the show, so the thought of being upstaged by Elena Gilbert seemed to drive her crazy. Their personalities seem to naturally clash so no one would ever expect them to be friends, but Katherine's hatred of Elena seemed to be completely over the top.
Maybe it's because she was jealous, maybe it was because she hated seeing someone who was so much like the weak human girl that she used to be, or maybe she just hated looking at someone with the same face as her. Regardless of the reason, Katherine seemed to hate Elena for sport.
Killing Jeremy
Many characters have senselessly died throughout the long run of The Vampire Diaries, however one of the most needlessly cruel murders in the history of the show has got to be when Katherine fed Jeremy to Silas.
Honestly it was just completely unnecessary and it seemed to be fueled by the fact that Jeremy was annoying Katherine in the moment, but this particular kill is a prime example of Katherine's complete lack of care and consideration when it comes to killing people, as well as her bizarrely over the top cruelty and brutality towards her doppelganger, Elena Gilbert.
Stealing Elena's Body
Obviously there is a bit of a running theme here when it comes to Katherine's shamelessness towards using and abusing Elena Gilbert. But the creepiest instance of Katherine's mistreatment of Elena is when Katherine literally stole Elena's body and tried to steal her entire life.
Katherine is a survivor above all things, but it seemed like she was at least attempting to turn over a new leaf when she knew that the end of her life was nigh. However, at the last gasp Katherine got cold feet, and she decided to install herself into Elena's body. But to make things worse, she didn't just take over Elena's body and bail, she decided to become Elena Gilbert.
Letting People She Loved Die To Save Herself
It's one thing to kill people you barely know as collateral damage in this seemingly endless smackdown between the most powerful vampires and supernatural creatures in the world. But it's an entirely different animal to let the people that you love die for your own sake.
Katherine has clearly been through a lot of trauma, and she had no way of knowing that Klaus Mikaelson would slaughter her entire family for disobeying him, but once that precedent had been set it seemed to become a habit with Katherine. Katherine developed significant relationships with people throughout her long life, but she never hesitated to throw them under the bus for her own benefit.
Toying With The Salvatores
Despite what the writers of The Vampire Diaries seem to want the audience to believe, it's actually incredibly creepy and weird to date two brothers. And it's infinitely more creepy and weird to date two brothers at the same time.
But, ever the spoiled brat, Katherine Pierce was more than content to be with Damon and Stefan at the same time, and even seemingly pit the brothers against each other in a weird competition for her love. She bailed on them once the situation got messy and dangerous for her, but as soon as they both started falling for Elena she did everything she could to once again become their object of affection and competition.
Toying With Everyone Else
Katherine is undoubtedly a master manipulator, and she presumably developed those skills so well because she spends a lot of time manipulating people, even when she doesn't have to.
Katherine put a lot of effort into bringing Elena to heel and giving Klaus every opportunity to break his hybrid curse, but until Klaus and the rest of the Originals showed up in Mystic Falls Katherine was the strongest and most powerful vampire in town. She really didn't need to bring the world down around the entire Mystic Falls gang just to hand Elena over, and she seemed to enjoy unnecessarily tormenting them.
Killing Caroline
Probably one of the most brutal things that Katherine ever did on TVD was killing Caroline. She used Caroline to send a message to Elena, Stefan, and Damon, and she showed no remorse in the process.
It was presumably unimaginably terrifying for Caroline to be killed by what she thought was her best friend, and it seems almost crueler to have killed Caroline knowing that she was going to become a vampire.
The idea was to torment Elena as much as possible, and she showed no regard for Caroline while putting Elena in a position where she may have had to make the decision to let one of her best friends die.
Leaving Stefan And Damon Behind
When Katherine initially showed up in Mystic Falls, she completely turned Stefan and Damon Salvatore's worlds upside down.
They were enchanted by her and fell in love, which pitted the two of them against each other. She used them as a tool against the threats that the Salvatore family posed against her, all while planning to turn them into vampires in the end anyway.
But once everything hit the fan, she left them holding the bag completely and allowed them to transition on their own while believing that she had been killed or was trapped in the tomb.
Surviving
The one thing that is the most memorable and shameless about Katherine Pierce as a character is that she is a survivor first and foremost, no matter what the cost. Katherine feels no guilt about the fact that she would let anyone die so that she could live, and there aren't any moral or ethical lines that she wouldn't cross for her own sake.
However, Katherine is also the epitome of a survivor in the saddest sense of the word. Nothing matters more to her than just staying alive, so everything about who she was had to be given up or stripped away in order to do that, but she never hesitated when it came to that either.