Absence of Malice details what can go wrong when a reporter becomes the story instead of reporting it. The protagonist commits two sins in the movie:
First, in reporting the story she becomes so enamoured with her subjects that she fell in love with him. This is unethical because in the course of reporting she lost sight of the story and her job. This immersion into the life of the subject clouds her and blinds her from making sound decision about Gallagher.
Second, because she is so imbued into the life of Gallagher that she is willing to disclose her sources and talk to Gallagher about the information that helps upend the case against him.
It is unfortunate because she was filled with passion and appeared to be a great reported that allowed her heart to lead the story rather the subject.
I agree with your analysis in which Megan began to lead the investigation with her heart, as opposed to with her mind.
By falling in love with Mike, she began to take whatever steps necessary in order to release his name from being involved in the murder.
It was sad because, as you said, she was a passionate reporter and did have a lot of potential.
Christina