Chickens are actually reasonably smart and social animals. They can learn tricks and everything.

How did any even survive at all? And wouldn't they have had horrible, obvious burns?

That scene traumatized me when I was a kid!

There's no reason to believe that roasting it alive is any better than chewing it to pieces. Quickly destroying its brain(s), however, would be much more likely to give it a "humane" death.