A lawyer unwittingly frees and later marries a guilty man. Now she?ll stop at nothing to see him convicted of a murder he didn?t commit.

Let me unpack the story as I understand it:

A lawyer helps a client beat a murder rap believing her client to be innocent [back story or opening setup].
Only to discover new and compelling evidence that he actually he did the crime [inciting incident].
But because? of the Constitutional guarantee against double jeopardy, he can't be retried [complication].
So she conspires to bring him "to justice" by a false charge of murder [plot].

Well, why not cut to the chase? She decides to murder him herself.? Get "street justice".

The dramatic challenge for you as the writer is: how to make the audience at least empathize with her motives? Well,? I suggest that one way is the nature of his crime.? It can't have been an impulsive crime of passion, done in a moment of anger (and under the influence of, say, alcohol).? Rather, his original crime has to have been premeditated and heinous,? say an act of prolonged painful torture on a helpless woman while she was alive, and depravity upon her body after she was dead.? ?In this way the narrative can sell? an audience on street justice emotionally? as justified even if logically they know otherwise. (There are any number of movies and crime novels that resolve the story with street justice.)

fwiw

And here?s one more take in under 25 words:

When an attorney discovers the innocent man she freed from prison is really a murdering psychopath, she throws her ethics aside to stop him.

Here?s another spin:

A corporate attorney, working pro bono, frees a man convicted of murdering his college sweetheart. But when his terrified new girlfriend finds strong evidence of his guilt, they hatch a plan to take him down.

Notes:

She?s working pro bono (I did this for 12 years on a murder case).

I?m dropping the married part, too much for 120 pages. There can be sexual tension.

I won?t reveal that the plan is to hide his new girlfriend who has been beaten to a pulp and set him up for her murder.

My lawyer?s ethics have gone out the window because I?ll give her a terminal illness. Only way I can see now to solve this problem.

I just have to make him beyond terrible for the audience to root for the women here.