It's not an excuse - it just is a fact that weak people take the easy way out, no matter who else it harms. Mangum has drug, alcohol and psychiatric problems. She isn't equipped to make good judgments or judgments that consider the needs and fairness of and to others, and that is her fault (maybe not the mental issues), not anybody else's fault. But they're still present and do affect other people adversely when she makes a bad decision, which is most of the time. It's up to herself and a cogent memeber of her family, if there is one, to straighten her out.
Because of her failures, and because Liefong is supposed to be an educated, public servant, I view him as having the role of the 'adult' in this matter, entrusted with advancing the people's best interests, not his own. That is a burden he chose, and it places on him the responsibility AND THE AUTHORITY to not have allowed her to advance this case beyond the complaint stage.
I don't think Nifong is weak in the same sense Mangum is. He is corrupt and places his own desires over the good of the people and the integrity of his office, but he is not weak in the sense that he can't control his decisions and actions in the way that Mangum can't. Mangum lives and acts in the moment, Liefong does not.