I haven't read the original but the followings are what I get
Don't read on if you want to watch the movie yourself
1. Most if not all people in Chicago did not know the existence of central governing body, separated by the two walls and the slum (Those captured had their memory erased)
2. She needed to fight back as any peaceful mean by negotiation had failed
3. Those are the defiant of central government, who had more contact with the truth
4. David used fractions to create conflicts amongst them so as to control Chicago under his regime. This was the experiment. However, a large scale war might not be the interest for him so he deployed the memory erasing gas to the population