I don't find it flawed as I think this murderess's brand of premeditation to be very indicatative of someone in complete control of their faculties. I strongly believe there is a huge difference between someone who buys a gun, and then kills someone (premeditated) and someone like JA who methodically planned for weeks, for both pre- and post murder behaviors. Add in the stalking, and hacking behaviors and it becomes apparent that every aspect of her relationship with Travis was "premeditated." Everything from singling him out at PPL, converting to his faith, harassing "rivals," hacking into his personal affairs, and making sex tapes which she more than likely tried to use against him.
If I were a juror, the only way I could see "mental illness" being a strong mitagator--b/c it has to outweigh the aggrivator(s)--would be someone who has multiple disorders, a low IQ AND someone who suffered documented abuse.
But that's just me!