Okay. I might be bashed for saying this but the truth is that I've always thought that Elena was sort of a Mary Sue. It's true. She has two brothers fighting over her. She's a doppleganger. In the first few episodes she's innocent and sweet. Hardly ever does anything wrong. And to top it all off she forgave the one person who basically ruined her life. She's a pushover! Not to mention that she has an evil twin and hogs the spotlight 100% of the time. She's practically perfect in every way. Which is why I think she should be the one that kicks the bucket instead of Katherine.
Katherine, on the other hand, is a fighter. When Klaus was trying to kill her she refused to go down without a fight. Although she had Elijah to protect her, she didn't need him. She didn't need to be rescued. Unlike Elena, who has both Damon and Stephan scraping for her attention. Katherine used her wits! She tricked a vampire into feeding her blood and then she made herself into one. She did what she needed to to survive. I don't think that she was balantly evil. She always had motives behind her madness. Each step she took was to ensure her own survival or to reclaim the love that she had lost. If she was allowed to exist I think that she would eventually be redeemable.
I mean look at Damon. He wasn't exactly a saint before he met Elena. In fact one of the worst vampires. Killing other vampires to save his own skin. Actually killing Elena's brother. You know just to name a few things off the top of my head. But he eventually turned over a new leaf. Only to turn it back over again... but you get my point. Anyone can change for the better.