The state doesn't need motive. I am sure when the facts are presented the jury or the judge will have their own idea why she did this....the reason doesn't really matter but is always nice to know.
Not being a great legal-mind, I have wondered this, myself. It seems that all they need to do is present the ample evidence, rebut the defenses arguements (again, with the facts), restrain the attempts of JB (etal) to entangle, twist, and confuse, the evidence, - and let the jury come to it's own reasonable conclusions.
In this case, I really don't think the defense is capable enough to successfully employ smoke and mirrors, the prosecution inept enough to blow the case, or a jury stupid enough to be manipulated.