After a former star witness is kidnapped by an escaped (or exonerated) killer she must find a way to stop him as he hunts down the jury members who convicted him.

Why has the witness been kidnapped and kept alive while the killer goes around and murders the jury who convicted him? Why does he keep her alive, or bring her along at all?

Actually, it is the same thing. The first half of the sentence was just flipped around, is all. The star witness still has to find a way to stop him as he hunts down the jury members who convicted him.

BEFORE ?

After a former star witness is kidnapped by an escaped (or exonerated) killer,

she must find a way to stop him as he hunts down the jury members who convicted him.

AFTER ?

After an escaped (or exonerated) killer kidnaps a former star witness,

she must find a way to stop him as he hunts down the jury members who convicted him.

I don't think your alternative is improving the grammar, as the sentence structure now suggests that the killer ... must stop him.

But hey, I'm only a philologist... :-P