It's a little clunky, in my opinion. The general plot you're conveying is good, but changing various words I think will give you something more strong and succinct. "Cold-blooded killer" could be replaced with "guilty" for example. The use of the word "chooses" seems to provide less of an antagonistic plot force, because it doesn't convey she's struggling to come up with a plan, but just bada-boom there's a plan. So if her coming up with a plan is really that simple, then I'd ax it because its not a major plot point. If the antagonistic force is executing the plan then that's what I'd focus on. Furthermore, how is wanting to put him back in prison, when he is in fact guilty, not ethical? Clearly, it must be because whatever her plan is is not ethical, but that's not super clear.
So: "When an idealistic attorney discovers the man she freed from prison is guilty, she struggles to execute a nefarious plot to put him back behind bars" would be my recommended logline.