Attitude toward humans
In 1864 she stopped and fed on the passengers of a coach showing Damon, before he was fully turned into a vampire, how it is done, taking sadistic pleasure over the coachman being so concerned about the safety of a lone lady on the road at night. She wickedly smiled at him, thanked him for his concern and then fed on him (Children of the Damned), clearly showing that all humans were to her, was food and amusement at that time. However, it is known that even then she could find humans as potential mates. It is a fact that she had a sexual relationship with both Salvatore brothers and eventually turned them both and saying to Stefan that she loved him as she was being carted away to be presumably entombed in the Church. This is likely an exception since she hasn't shown any concern for human life even of the opposite sex presently.
In this century she used Isobel Saltzman as an agent in her plan to destroy the tomb vampires in Mystic Falls most likely knowing full well Isobel's cold disregard for human life and her willing to use them to murderous ends. Still in today's world it is unknown if she hunts humans or at least drinks human blood without killing or even attacking them like Lexi. She was first seen in the 21st century when she kissed Damon when she was dressed as Elena Gilbert letting him think she was Elena. She also let John Gilbert think she was Elena as well until she hacked off his fingers and stabbed him with the intent to kill him. (Founder's Day) She was interrupted by Elena entering the house and finding him. However given her merciless attitudes it is likely that she does hunt humans (although this hasn't been depicted on screen).
As Elena called 911 Katherine reappeared behind Elena unseen by Elena despite Gilbert's gasped warning. She didn't kill Elena or reveal herself to her but toyed with her using her super speed. However her attitudes toward humans haven't changed. As a bait to create a crisis between the Salvatore brothers and herself after Stefan rejected her, she deliberately smothered Caroline Forbes in her hospital room, a human that did her absolutely no harm, knowing that she had Damon's blood in her system.
However while she has no regard for human life she doesn't care for her fellow vampires either. She admitted to Stefan Salvatore that she betrayed the Mystic Fall vampires in 1864 by making a deal with George Lockwood, the son of the mayor of the time and a werewolf, ostensibly the vampire's mortal enemies. In exchange for covering up his own attacks on humans, Katherine allows him to blow the whistle on the town vampires so he can increase his standing in the Mystic Falls founding families and in his personal family. She did this so she could be rid of her personal enemies among the vampires and fake her own death. It was him who arranged to have Katherine escape the tomb in exchange for the Moonstone, which at the time Katherine had in her possession. This shows a ruthlessness that demonstrates she is out solely for herself, despite the fact that she cares about Stefan.
Katherine's ruthlessness and indifference to others is likely due to the fact that her family disowned her for having an illegtimate child. Also the fact that Klaus wanted to use her as a sacrifice for breaking the sun and the moon curse didn't help matters making her all the more bitter and distrustful.
* She, Isobel Saltzman and Rose are the only characters thus far to have an episode named after them.
* Nina Dobrev who plays Katerina Petrova/Katherine Pierce, was born in Bulgaria and speaks Bulgarian.
* Katherine at 518 years old (since her transfiguration at about 15 to 18 years old) is presently the sixths oldest vampire to be revealed. Elijah, Rose and the recently killed Trevor all predate her although it is still unknown when exactly they were turned into vampires. Pearl, until now, was the oldest revealed. At this time she could be as much as the second oldest revealed. She was transfigured roughly in the year 1434 approximately 576 years ago. Her daughter Anna if she was truly her biological daughter would have been second, transfigured no later than 1451, 560 years ago and 17 years after the last possible year Pearl was still a human capable of having children and thusly could be as much as the third oldest vampire revealed on the show, again depending on when Rose and Trevor were turned. Lexi, was transfigured in 1660 making her 349 and thus was the sixth or seventh oldest introduced.
* Rose turned Katherine, which was Klaus in the books, completely changing the backstories of Katherine and Klaus for the series.
* Lost Girls (flashback)
* Children of the Damned (flashback)
* Blood Brothers (flashback)
* Founder's Day
* The Return
* Bad Moon Rising
* Memory Lane
* Kill or Be Killed
* Plan B