They intended all along that the people inside die

I think Sabine always intended that the people inside the isolated area would perish. She is only going through the motions of saving people inside. When she says "No one in, no one out," she means permanently. That is why she was so quick with the snipers and so careless with the food drops. That is why she hasn't put thought into controlling the violence or crime inside, or providing medicines for other diseases. And why she is so dead-set against anyone going inside for any reason. From her perspective, the moment the virus started spreading everyone inside the perimeter was already dead.

Think about it. What is her exit plan? She did not set up any means to establish when people are no longer being infected. She is not asking for updates on infection rates. When a child was discovered who could be the cure, the first thing she said was "Get him out." Her plan all along was to let the disease run its course; ie kill everyone it can and die out.

Also notice that when she was personally trapped on the border of the perimeter, she started talking about her life and death. She shared how compassion led her mentor to die getting close to a patient. So her approach is cold blooded, separate the affected area and let them die.