How far north/west/east would you have taken those buses, without being able to communicate where they were headed, without anything waiting at the end of the line, and without the ability of those buses to be even modestly self-sufficient (no toilets and no under-the-bus storage areas that could be stocked with food/water)? Once the first evacuees step foot on that bus, they become the responsibility of whoever is running that bus.
I will fault and am faulting Nagan for not having stocks of food, water and portable toilets ready to go at the Superdome/Convention Center/et al. I'll fault him for not having flood-resistant communications for the police. I'll fault him for waiting until Sunday to use the word "mandatory" next to the word "evacuation". I'll fault him for not geting those buses out of flood-prone areas so they could help evacuate the "refuges of last resort" after the storm. But, unless he had someplace outside of New Orleans to take those buses, I can't fault him for that.
That all may be true, but you left out the most important thing, at least they would be alive.
"But, unless he had someplace outside of New Orleans to take those buses, I can't fault him for that."
That, again, would fall to the plan that should have been in place and executed by Mayor Nagin. Even without a plan, people on the road in a bus could have been gotten to just by driving up next to them, instead of trying to figure out how to get to them in a giant toilet bowl.