The Vampire Diaries had a variety of villains throughout the years. However, few were as memorable as the elusive Katherine Pierce and Klaus Mikaelson. Katherine and Klaus were enemies themselves, which added another level of intrigue to the dynamic they each had with the main Mystic Falls gang. Katherine and Klaus had both been known to create havoc in the small town. Having had years more experience than the other characters on the show, they tended to be more intelligent and several steps ahead of their opponents.
As annoying as it may have been for Stefan, Damon, or Elena to create an alliance with either of them, sometimes it was necessary to stop a more significant threat. But, it was never safe to entirely trust either of them, as Katherine and Klaus were known to be focused on their agendas first.
Klaus: Nearly Impossible To Kill
Klaus Mikaelson was not just an Original Vampire; he was also the only Original Hybrid. Due to Vampire and Werewolf blood running through his veins, Klaus was practically unbeatable. He was harder to kill than any of his siblings, and since there was nearly nothing that could destroy the Originals, that said something.
However, it was not just Klaus' status as a Hybrid that made him difficult to execute. The heroes of The Vampire Diaries discovered that killing an Original Vampire causes the rest of their sire line to die too. Since all the main vampires on the series are under Klaus' sire line, to kill him would mean the demise of them all.
Katherine: It Was Personal
Katherine had a long history with Stefan and Damon Salvatore. After getting each brother to fall in love with her, Katherine fed them each her blood before they died, then abandoned them in 1864. While Damon and Stefan both believed Katherine had been stuck in a tomb, they discovered Katherine had never been trapped during season two. Instead, Katherine had been on the run, occasionally checking up on Stefan through the years.
However, it not only just personal with the Salvatore brothers, as Katherine also saw a threat in Elena. Even though Elena could never fight back physically, Elena had the life Katherine had always wanted.
Klaus: A Family Of Backup
Klaus was not the only Original that Mystic Falls had to worry about. Soon after learning about Klaus, his siblings Elijah, Rebekah, Kol, and Finn appeared in town also. The Mikaelson family was not always in agreement, and their issues became a problem for Elena and her friends.
While Elijah and Rebekah had each conspired against Klaus, they would eventually end up on the same side of a threat. Klaus, Elijah, and Rebekah's vow of "Always and Forever" kept the three of them linked, as well as Elijah's desire to keep their family together.
Katherine: Master Manipulator
Katherine had a long list of talents. Among the top of the list were survival skills and manipulation. Whether it be convincing people she was Elena or tricking them into doing her dirty work, Katherine had a way of getting what she wanted. Katherine could manipulate nearly anyone, and was often steps ahead of the Mystic Falls gang.
While they were trying to figure out what game Katherine was playing, she was ahead of them, manipulating various others for the grand scheme of her plan. It did not end at managing people. She could also manipulate the situation to her advantage, a talent that brought problems to everyone else.
Klaus: Turning Off Stefan's Humanity
There were plenty of things Klaus was willing to do to get his way. One of them included returning Stefan to his Ripper state. To do that, Klaus forced Stefan to turn off his humanity and go with Klaus on his journey. With his humanity off, Damon and Elena had to work together to find Stefan and bring him back.
By turning off Stefan's humanity, Klaus forced Stefan to do many things he would not have usually done, causing a massive amount of guilt onto Stefan's shoulders when he turned his humanity back on. Klaus wanted to be reunited with his friend Stefan, who he had connected with during the 1920s.
Katherine: Dangerous As A Human
Without her vampire abilities, Katherine still found a way to be a threat. Katherine had Traveler's Blood, which allowed her consciousness to jump into another person's body. As she lay dying as a human, Katherine pulled off one last trick. Katherine prepared herself to die and then be sent into Elena's body.
In Elena's body, Katherine tried to convince everyone she was Elena and also tried to get back together with Stefan. Katherine's plan did not work out in the long run, but everyone should have known never to underestimate her.
Klaus: Daggered His Siblings
Klaus could be very short-tempered and paranoid. Due to those traits, if Klaus' siblings went against his plans, he had a way to subdue them. Klaus would dagger his siblings, desiccating them for decades or centuries at a time until he decided their punishment was over. At which, Klaus would take the dagger out.
While this is something he would only do to his family, it gave the rest of the characters a reason for concern. If Klaus was willing to do such harm to his own family, it was terrifying what he would be willing to do to them.
Katherine: Caroline And Jeremy
On two separate occasions, Katherine killed two people closest to Elena. While Caroline transitioned into a vampire, Jeremy died as Silas drank him dry. In an instant, Katherine had changed Caroline's life forever. A welcome twist showed Caroline thrive as a vampire.
In Jeremy's case, he would not be revived immediately. Jeremy's death brought out the Humanity-less side of Elena. It would not be until "Graduation" that Bonnie would successfully bring Jeremy back to life, but lose her own in the process. Katherine was never careful with people Elena cared about, and these were two instances where she proved how ruthless she could be.
Klaus: Was Willing To Sacrifice Anyone
When Klaus finally had the opportunity to unlock his werewolf side after centuries, he was willing to sacrifice anyone to get it done. Klaus worked to his selfish needs, putting other people's lives at risk. Klaus could be drastically unpredictable with his victims, while also knowing who he could hurt to get vengeance.
When Klaus was trying to create other Hybrids, he willingly hurt several werewolves to see if any would take. Klaus later risked Tyler's life to see if the Tyler would successfully turn into a Hybrid. Klaus would not hesitate to kill someone, or a person close to them if they were conspiring against him.
Katherine: She Knew Them So Well
One of the most dangerous things about Katherine Pierce was how she picked up on things. She was observant, and with that grew to understand the ins and outs of nearly every main character. Katherine knew Stefan, Damon, Elena, Caroline, Bonnie, and Matt so well that she could anticipate what they would do.
She knew how to manipulate them into doing what she needed and used that to her advantage. Katherine knew that if she linked herself to Elena, Stefan and Damon would protect her life to protect Elena. Katherine understanding of the Mystic Falls gang so well was not helpful to them as they tried to defeat her.