But insects don't need head to kill you. Even without a head their body can still move around and will perform its last movement order. So in order to kill a mantis, you cut off the blades first. I can understand that cutting off cockroach's limbs will be too much trouble, mantises have only two blades.
Well, he did cut off some of its blades, so it's not too bad. But certainly do not aim for the head first. At least with the head still intact, the mantis would be dazed enough by the loss of its blades that cutting off the head after that will be much easier.
Anyhow, thanks for bringing this back.