A civilian does not lose their protected status, their immunity from attack merely because they enter an area that they're not allowed or that they're told not to enter.
The only way in which people lose that immunity from attack is if they directly participate in hostilities.
There needs to be a legitimate military objective and operational objective – and the only way to achieve it would be to destroy the civilian property. And so, at that scale, that's simply not quite plausible.
A kill zone is in essence the announcement of a party to the war that they won't take feasible precautions, that they won't verify the status of an individual before attacking them. And that definitely violates international law.
Simply being present in a certain part of a combat theater does not amount to active participation in hostilities. And only active participation in hostilities makes a civilian lose their protection under international law.
Unlike many other combat zones in the Gaza Strip, the annihilation of infrastructure and buildings in the perimeter sometimes occurred after the area was captured, when no immediate or concrete threat to the forces was present.
