His supreme error, IMO, has been in fighting a running defensive battle against the walkers without any intention of actually solving the walker problem. He has failed to realize his role in this apocalypse. Zombies are a disease and he and his group are the cure, the white blood cells, the killer T cells of the world. He should have taken the battle to them and destroyed them wholesale wherever they occurred. He should have been thinking of ways to kill them by the hundreds of thousands.
Wow, that's pretty intense insight. What would you suggest to him on how to implement your strategy?