At the end of Taking Lives
we see that it was James Costa aka Martin Asher who is the real killer. But by that time this is discovered he has had sex with Scott, has taken another life and has escaped. Scott is terminated from her service at the FBI for her conduct. In the last scene, few months later, we see Scott, pregnant, coming from a store to a secluded house living all alone. Asher/Costa arrives there and abuses and threatens her to start fresh with him. Finally he loses his mind and stabs Scott in her belly, but Scott pulls back the scissors and stabs his chest revealing that this whole thing was staged(she isn't pregnant) to nab Costa/Asher. Scott then tells Leclair about their success.
My question is if this whole sacking of Scott and her living secluded, pregnant, was a set up to catch Asher/Costa, as he would come to her, How can Scott kill him? She is bound by the law, this whole thing was a setup by the authorities. She expected Asher/Costa to abuse her when he came and possibly attempt to kill her, but how can she kill him and get away with the law. He is not a convicted murderer. He hasn't undergone a trial.
Also, Leclair must have known that Asher/Costa would try to harm her, so why didn't he have an arrangement to catch the killer alive. Is this setup and most importantly, the killing part, legally justified?