Well, first of all, napalm and flamethrower are two different beasts. The flamethrower projects a flammable liquid; napalm is essentially gasoline in a jellied form. They would behave differently, and napalm would tend to burn longer, I should think.
We must also consider the nature of zombies. As has been pointed out, it’s tough to cremate a human, being mostly made of water. Are zombies physiologically any different from live humans? Brooks says no, but in the original Night Of The Living Dead, a sheriff is heard to say “…burn 'em, they go right up.”
My opinion: We may assume the sheriff was talking through his hat; I know for a fact that Brooks did more research than the guy in NOTLD.
Brooks discourages burning the dead, as they tend to shamble around a while while burning before de-animating. Naturally, this is dangerous to you, me, and anything flammable near the zombie.
We may take into account, though, exactly what fire does to the outer body, as explained by gabriela with no small eloquence. If the front of the zombie is hit, either by napalm or flamethrower, that zombie is going to be blind if it wasn’t already; the surface of the eye will dry out and the eyes will probably burst from the sudden drying effect and the heat. The sense of smell will be shot real quick, as the zombie must inhale to use it, thus frying his olfactory sensors. The sense of hearing will last somewhat, as the eardrums are located inside the head, but I’d imagine the zombie isn’t going to be hearing much aside from the crackle of flame and the sizzling of his own muscle tissue.
Sense of touch goes quick, as the skin loses its nerve endings in third degree burns.
This leaves the sense of taste, which I don’t imagine would be much good for navigation, much less locating and tracking prey. Unless that zombie has some sort of paranormal “life detector” sense, his hunting ability is pretty much … um… toast.
Since any human can quickly detect and avoid a burning zombie (or let him have it upside the head or spine with a ball bat), we can say that this method renders the zombie incapacitated for all practical purposes.
…aside from the danger to structures, naturally.
And what delivery method are we using for the napalm? Air drop? That sounds kind of scary to people stuck in buildings at the target site. Or are we carrying it around in buckets to throw on the zombies?
More information would be useful.
