Not doing the Manhunters correctly is in keeping with the way they are mangling super hero movies for the big screen to make them easily understandable. The entire "robots gone bad" thing is something that has been drilled into the heads of the public to the point where they figure they can sell that without needing to bother with the entire subtext of what gives this paticular idea personality. As was pointed out, the robots in this case didn't exactly go bad in any absolute sense, and they WERE screwed over by the council which makes the entire situation fairly tragic. On a lot of levels the Manhunters can be considered fairly heroic in their own way.
I *DO* understand that it's difficult to translate years worth of subtext and concept building into a two hour film, especially given the video/audio nature of the medium and it's limitations compared to text, or even sequential art in terms of delivering depth and exposition. But then again a lot of the problem also comes from the writers/directors wanting to put their own mark on things, and hopefully get a hybrid franchise going, so they inevitably want to focus on origin stories, yet toss out cool things that wouldn't have come about until years or decades after a character was established. For "The Manhunters" to work you shouldn't be having a newbie Hal meeting them, the whole idea is the big surprise that "yes, there was an attempt at galactic policing before the Corps.".
As far as the robots not being able to see shades of gray, yes and no. It's been stated that way, but I think the issue is that they tend to enforce a set of codified laws accross the entire galaxy with themselves as the ultimate authority, as opposed to by and large working with the local/planetary laws and authorities, and frequently within the customs of that planet. The Manhunters might be doing the right thing, but at the same time nobody likes robocop showing up from on high and say arresting all your local officials or whatever.
In the case of a Manhunter arresting both a bank robber and the hero that hit him, that one is actually pretty straightforward since they are both criminals in an absolute sense. Heck, most heroes spend a LOT of time hiding from the authorities themselves, being vigilantes. The whole point of them concealing their identities is to avoid accountability for their actions, putting stopping the bad guys ahead of things like collateral damage. The idea being that the mask/secret identity means nobody is going to arrest him for assault, breaking and entering, destruction of public property, and any civil matters that might arise. The big differance is that a Manhunter *CAN* arrest a DC analogy to say a Spider Man or whatever, along with the villains, as opposed to being as powerful as the cops are. Unlike the Green Lantern Corps. it also isn't going to be following the policies of the planet any more than the hero or villain would in order to do it, because it's enforcing the laws laid down by the council and views all other laws as being irrelevent.
That's how I understand it at any rate. The concept also reminds me a bit of say "Watchmen" or "The Incredibles" where due to complaints about registered heroes (which is again, why most heroes are vigilantes as oppoed to goverment operatives to begin with) they decided to ham-handedly try and just get rid of them rather than dealing with the more complicated, (and expensive) matter of making things work. As the Manhunters were created to be self aware being basically discarded was an especially douchebag thing to do. The Guardians are kind of asking for it in this paticular case, though it's no fault of the corps who simply succeeded them without the knowlege to begin with.
Ah well, enough rambling. Seems like a banal game based on shallow movie logic, throwing out Manhunters simply for being fairly iconic to the franchise.
As far as doing a good Green Lantern game goes, I think it could be done, but really the only system right now that could pull it off might be the DS or it's successor. While not appealing to the lowest human denominator, the whole "Green Lantern" concept is perfect for a turn based RPG for a system with a stylus. It would allow you to draw pictures with the stylus which would reflect the manifestation of the powers on the main game screen, with stats largely guiding effectiveness, and of course concepts like energy and willpower influancing your drawing as effects that are going to be larger and more powerful would drain the charge of your ring faster (like spellpoints, various aspects of willpower would of course influance things like damage, and the size of objects (for AoE), the strength of shields, and similar things, and of course there would be issues like debuffs from things like fear that influanced your stats and affected your power manifestation.
I could see a good Green Lantern game done around mechanics like those above, but that isn't quite the flashy "style over substance" approach they want with a movie tie in game, and of course while a fairly accurate way of acheiving what you see in the comics, turn based RPGs are currently a niche genere.