The DA I'm sure is reluctant to take on a case this weak. I mean, we're all pretty sure what went down and that she was involved to some degree, but proving that to a jury beyond a reasonable doubt when the kid is wailing and crying on the stand and the bad guy is insisting she had nothing to do with it would be tough.
It would also jeopardize their prosecution of the main offender.
I don't know how they do things in those parts, but my money's on her having made a deal to testify in exchange for non-prosecution of what amounts to a fairly weak case. If so, this will come out when Ludwig's lawyer files a "Reveal the Deal" motion.
The case against Kara may be weak, but it would surprise me if the prosecutors feel they need her testimony in any way.
They have plenty of documentary evidence of her testimony (all her email, blogs etc.) that could be admitted in some circumstances.
But more so, unless they have a problem with admissibility, they have a rock solid confession from Ludwig, found the murder weapon where he said it was (under the front seat of his car), etc.
They just don't need her testimony as far as I can see, so they have no incentive to deal for it.
If they made the decision not to charge her, I doubt it had anything to do with wanting to deal.
I believe that the D.A, did not charge her because if he did charge her, by the time her lawyer got though, the jury and citizens of the County would have been ready to lynch the D.A.