"There's one thing I don't understand about pinkamena art, why do people always draw her with straight hair? She's not depressed when slaughtering...if anything, she's happy as hell."
Most people seem to be unaware of the significance of the timeline and the difference between the two non-canon Pinkies/Pinkamenas:
Cupcakes Pinkie Pie: Cupcakes was written before either "Party of One" or "Cutie Mark Chronicles" aired and, at the time it came out, she was depicted with her normal curly hair. "Cupcakes Pinkie" is a sadistic psychopath, basically just a psychopathic variant of her canon self.
Pinkamena: Pinkamena, with straight hair, is a classic depressive schizophrenic with comorbid MPD (Multiple Personality Disorder) tendencies. It is interesting to note that this is very much in line as a valid possibility for her canon character who, as shown in the show, displays very strong characteristics of Schizophrenia: Delusions, Disorganized speech, and Grossly disorganized behavior, as well as Manic episodes characteristic of Schizoaffective Disorder. And it can also be said that her "Pinkie Pie" personality is a secondary personality that appears during her mania, while her "Pinkamena" personality should be considered as her "true" personality, as it was how she was as a filly.
Now to your question: Although her behavior in "Party of One" was mostly passive or harmless, true schizophrenia, over time, usually culminates in psychotic episodes which are often destructive and violent, and her violent retaliation against Rainbow Dash shows that she has the propensity for such a prognosis and was likely on the verge of a psychotic break - but fortunately deviated toward catatonic schizophrenia instead.
So, in short, the straight-haired Pinkamena characterization is highly fitted to the role of a vengeful psychotic - but she would not likely be laughing or smiling while performing in this capacity, but rather screaming or crying - and this is the personality of hers that I adore most.