|“||I did find your birthplace. And I slaughtered your family. I guess we’re cool.||”|
The relationship between Klaus and Katherine Pierce has been antagonistic. In the beginning Klaus was kind to her, but later when Katherine discovered that he needed her for the sacrifice, she ran away. She killed herself with Rose's blood in her system. Because she turned into a vampire, she became useless to Klaus. Klaus's revenge was to kill Katherine's whole family.
They met in 1491, after Katerina was exiled from Bulgaria, she went to England and came across Elijah and Klaus. As soon as she found out that they were vampires she ran away and took the moonstone with her. She became a vampire, by having fed on Rose's blood. Later, Klaus found her birthplace (Bulgaria), he slaughtered her whole family, because she ran away from him.
In The Last Dance, Klaus who still was in Alaric's body, gets information on Elena and her friends. He then compels Katherine to hurt herself by sticking a knife in her leg and repeating it again and again. Later on, he asks her for her opinion on what shirt to wear to the dance, he gets surprised by the many weapons found in Alaric's Apartment and asks her who Alaric was.
In The Last Day, Damon asks Katherine where Klaus has kept Caroline and Tyler. She replies by giving him the address. Klaus returns to Alaric's Apartment and is immediately suspicious of Katherine. He compels her to tell him what she has been doing but, as she is taking vervain, she is able to lie. He is still suspicious and 'compels' her to take off her bracelet, walk to the window and stand in the sunlight. Katherine has to do it and screams. Eventually Klaus believes she is compelled and says she needs to do something for him. Klaus later compels Katherine to phone Jenna so he could get his vampire for the sacrifice.