Other than Katherine Pierce, Klaus Mikaelson was probably longest-lasting villain on The Vampire Diaries. He was so unequivocally evil and twisted that it was almost a pleasure to watch him as he manipulated, tortured, and played with the residents of Mystic Falls.
One of the reasons why Klaus was so popular with fans despite all the hurt he caused, was that he was just so charismatic. He was also incredibly compelling, as his backstory was far from happy, and his complicated relationship with his siblings even went on to spawn a whole new show. Klaus inflicted a lot of pain, but none more so than unto himself.
Felt Bad - Every Time His Siblings Turned Against Him
Honestly, Klaus was absolutely devastated every time Elijah or Rebekah, or sometimes both, stood in his way. He was extremely close to those two growing up, but as the years ed, Klaus grew more and more bitter and hardened.
All Klaus ever wanted was the and love of his siblings, which seemed reasonable, but a lot of Klaus's schemes were completely psychotic. Therefore, sometimes Elijah and Rebekah helped put a stop to his plans, and this devastated Klaus, as he felt his own family were rejecting him.
Hated - When He Possessed Alaric
Possession is an all too familiar trope within the science fiction and fantasy genre, as it allows actors to flex their acting muscles and it gives the characters that they play something different to do. It also hooks viewers in, as it can be entertaining to watch.
Klaus possessing Alaric, however, was not one of those times. It just appeared to be a rather futile way of retaining the mystery of who Klaus was. Matt Davis did a sterling job, but Klaus lacked his usual charm, and it was a bit of a rubbish way to introduce a villain as great as Klaus.
Felt Bad - When Caroline Used Him
The sizzling, underlying chemistry between Caroline and Klaus is one of the best things to come out of The Vampire Diaries. Neither party really ever acted upon their natural connection, which is what made this non-relationship so compelling.
However, perhaps it was better that Caroline and Klaus never became official. Sure, Klaus was an awful person, but Caroline wasn't exactly an angel. She knew how Klaus felt about her and used that to distract Klaus while her friends plotted against him on multiple occasions.
Hated - When He Turned Stefan's Humanity Off
At the beginning of Season 3, Klaus had successfully broken his curse, skipped town without any lasting damage, and also 'persuaded' his old friend, Stefan Salvatore, to come along for the ride. However, Stefan's friends weren't going to give up on him without a fight.
In order to get Elena Gilbert off their case, Klaus took Stefan back to Mystic Falls to say hello to his old friends. Of course, because this was Klaus, he also compelled Stefan to switch off his humanity, thus turning him back into a Ripper.
Felt Bad - When Kol Died
Although Klaus was close to Elijah and Rebekah, that same familial bond didn't particularly extend to the rest of his family. He despised his father, looked down on Finn, and treated Kol with something akin to indifference.
Despite that, Klaus was closer to Kol than he usually let on. The two brothers had the same temperament, and loved to cause misery and chaos to others. However, when the Gilbert siblings killed Kol, Klaus was completely devastated, and even tried to kill his precious doppelgänger, Elena, for revenge.
Hated - When He Bit Caroline
After Kol's death, Klaus turned a bit murderous, and so Bonnie trapped him inside Elena's living room with a spell. While the rest of the gang was off trying to find the Cure for vampirism, Caroline and Tyler Lockwood were in charge of watching Klaus.
During this time, Klaus was clearly still hurting. He wasn't thinking straight and so he bit Caroline, condemning her to a painful death. This was problematic, because Klaus claimed to love Caroline, and yet he was willing to hurt her to prove a point to his enemies.
Felt Bad - When His Father Beat Him
As mentioned previously, Klaus did not have the best of starts in this world. Once upon a time, he and his family were just a normal bunch of people, (almost) as human as they came. However, after one of his brothers died, Klaus's mother created the first vampires from her family.
This included Klaus's father, Mikael, who was not what anyone would call father of the year. He was an aggressive bully, but notably picked on Klaus because he was the product of Esther's affair with a werewolf. Mikael's disgust with Klaus would come out in horrific and violent ways.
Hated - When He Killed Carol Lockwood
Tyler and Klaus had developed quite a feud by the time Season 4 of The Vampire Diaries came around. After Klaus had sired Tyler in the previous season, Tyler had worked extremely hard to break the sire bond so that he could defeat Klaus once and for all.
In order to do this, Tyler went and found all of Klaus's newly made hybrids and broke their bonds so that they were no longer loyal to Klaus. When Klaus found out, he was so enraged that he killed Tyler's mom, Carol, in retribution, in a truly shocking and heartbreaking scene.
Felt Bad - When He Was Cursed
The curse of the Sun and the Moon was a predominant theme that ran throughout the second season of the supernatural vampire drama. However, there was a twist. Rather than the curse freeing a whole race of supernatural creatures, it only freed one man.
Naturally, Klaus was that man, and the curse was placed on him hundreds of years ago to suppress his werewolf heritage. The awful thing about this was that it was his mother who had cursed him. Everyone should be allowed to be themselves, and should never be diminished by other people.
Hated - When He Killed Jenna
For the first two seasons of The Vampire Diaries, Jenna acted as the mother figure for both Elena and Jeremy after their parents died shortly before the first episode of the show. She was very young, but she took on the role of their carer and did a magnificent job.
To make Jenna even more likable, she and Alaric started dating, and they were just utterly adorable together. However, when Klaus came along, he kidnapped Jenna and turned her into a vampire, only to sacrifice her moments later in order to break his curse.