Credit: The CW

At first glance, Klaus (Joseph Morgan) and Katherine (Nina Dobrev) seem to have a lot in common. They’re both ancient vampires who have a proclivity for murder and turning humans into supernatural beings. Yet these two are like oil and water on The Vampire Diaries. Actually, they’re more like sodium and water: Put them together, and things are bound to explode. 

Yet, when they met in the 15th century, Klaus was all hand kissing and courting. How did things go so sour for them? Well, Klaus knew that Katherine was a Petrova doppelganger, an essential ingredient in the curse-breaking spell he’d been searching for over the past few centuries. It would unlock his true hybridism by giving him access to both his vampire and werewolf powers.

When Katherine learned about Klaus’s supernatural side, she fled with the help of a vampire named Trevor. But she only truly escaped by committing suicide with Rose’s vamp blood in her veins. This caused her to transition, rendering her doppelganger blood useless to Klaus. Not realizing Katherine had given birth to a child — a lineage which would eventually lead to Elena (Nina Dobrev) — Klaus murdered Katherine’s entire family as revenge. He thought she’d squandered his final chance at breaking the curse, though Katherine’s baby escaped because her parents had sent the child away shortly after being born.

As evidenced by Klaus’s instructions that Katherine stab herself repeatedly in Season 2, Episode 18, “The Last Dance,” the Original is still a little sore. “I searched for you for over 500 years. Your death is going to last at least half that long,” he tells her.

Apparently immortal vampires can afford to hold grudges for a very, very long time.