The easiest target is the highest in command in a situation like this, but it is not necessarily or even actually the position to blame. FEMA has been blasted with criticism by senior members of congress since the early 90's. "The most bureaucratical bunch of prettythangholes I've ever had to deal with" is pretty close to the quote I remember from 1989 from a senior senator.
There is a need to blame someone. It makes those that are suffering feel better. The blame rests on the entire nation. Pick your natural catostrophy, be it flood, famine, earthquake, hurricane, tornado, drought or locust. The situation as it is now has shown the charity of the nation and the world. There can be no reasonable expectation of government to protect us from, or have prescience of a natural disaster of this scale.
The blame for the destruction is on Katrina. The blame for the suffering is on Katrina. The blame for the slow reaction time is on a fat, bloated bureaucracy of a government that the people that are blaming the government for their suffering want to increase.