Tough one in a way. The Declaration of Independence, a federal document says life is an inalienable right. Therefore can the feds supersede in a case where many lives are at stake.
Would the federal government supersede if a state government was actively killing innocent citizens. I would say yes. The only thing different in New Orleans was that the state government was passively killing its citizens.
In most cases it would be mute insofar that a competent state government would accept the help from the feds when the state was over their heads and needed resources etc. The nation has never seen such incompetence on such a grand scale. Katrina was the natural disaster. Louisiana politics are the national disaster. Truly is incredible.
I wonder what would have happened if the LANG & state police had refused to recognize Blanco's authority after she jumped the shark. IOW, they should have ignored her unlawful orders to prevent people from bringing in needed supplies. From another post, the Wildlife Service should not have been preventing private citizens from evacuating people from hospitals, but instead they should have been joining their brethren in pushing the witch out of the loop. All of them could have cited Nuremberg as the precedent for their actions.