Why she's always playing the b_i t_c_h of the story?
I saw some movies of her... and 90210, and she's playing the b_i_t_c_h all the time.
What's up with that???
The answer to your question basically comes down to two things
1) How the target audience relates to a given character:
In every story (be it a movie or television show) there has to be a protagonist and an antagonist. The protagonist has to be someone that the target audience (in AnnaLynne's case young women) will relate to and the antagonist has to be someone who they would see as a possible rival/threat in real life. Establishing this balance between the protagonist/antagonist is obviously a huge part in the success of any television show or film.
AnnaLynne McCord because of her looks and the fact that she is what most people would term as uniquely beautiful is not a very good candidate for the role of protagonist (which incidentally is the other role you are asking about: the girl next door, the nice girl, the girl who the audience can relate to) because how many girls do you know look like AnnaLynne McCord? I'd wager not that many.
So instead in the case of "90210" they make her the antagonist and cast someone like Shenae Grimes (who more ordinary looking) as the protagonist because she is far more in step looks wise with their target audience.
2) The need to make beautiful people fallible
The second part of this equation is something that has been going on in Hollywood for decades and that is the theory that if character is being played by an actor who is extremely attractive than the writers need to do something to make that character fallible (or flawed) or else the character will be totally despised by the audience.
The bottom line is at least on screen nobody wants to see someone as good looking as AnnaLynne McCord get all the breaks in life. So in the case of the characters AnnaLynne plays she usually has some type of flaw. She is either bitchy or she is wounded or whatever. Something, anything to make her more interesting to the target audience where they will get caught up in the characters storyline instead of spending the whole time being jealous of someone who has both perfect looks and the perfect life.
I hope I answered your question.