I, too, think Milton torched the walkers, but why? It didn't destroy them, and arguably makes them a little scarier looking. Apparently they couldn't move anymore, so I guess it was effective as far as preventing the Governor from using them in an assault, but why burn them? I guess it had to be Milton; anyone else would've just shot the walkers. Only Milton would want to destroy them from a safe place.
he probably never set a bunch of zombies on fire before so he didn't know they weren't going to die. I certainly expected them to be dead.