I can only speculate about her motives. But the system that is set in such a way that prisons can be privately owned and prisoners exploited for personal gain is a recipe for disaster. Stakeholders making profit from prisoners have a strong incentive to somehow ensure that as many people as possible get sent to prison for as long as possible no matter whether they are guilty or not. Some might decide to achieve this by bribing someone in the chain of obtaining/evaluating evidence. In a penal system like this it's just a matter of time before something like this happens.
BTW, it might also be a nice way to explain why punishments tend to be so much harsher than for example in Europe - in the US can being 'tough on crime' be profitable.
Massachusetts doesn't have private prisons, so I doubt that's the problem here. It's more that putting people behind bars is politically good for prosecutor with ambitions. Martha Coakly was almost elected Senator despite or maybe because of her role in a literal witch hunt.
It could also be purely ideological or psychological. It might give her a kick to have that power or she thinks if police suspects someone they are probably already guilty and deserve to be locked away and punished.
It would be an interesting study to see if she had access to pictures, gender, race, income, age of those whose results she forged.
