I think your all missing the point, yes mantids, just like a lot of predators kill and eat other living things, some are quick killers, Crocs, peregrine, pythons and boas etc some are slow, mantids etc, mantids dont care, if its no threat it will be eaten any old way, arse first quite often, if it is bigger and potentially dangerous prey a mantis will suspended it in space and make a concious effort to remove its head or legs to make the prey impotent.
( uk ) Its not illegal to feed live food invertibrates or vertibrate animals to predatory "pets" but it is illegal to cause unnessasary suffering to any vertibrate, you can be had for any animal cruelty and prosicuted (dont know the laws on this in the US) and can be banned from keeping animals. The video of the fish would be good evidence of cruelty over here. The cornsnake and mouse and other animals that are shown being eaten slowly by mantids would also be considered cruelty imo and if staged in this country, they are all or at least most of them staged in arenas for film despite how natural it looks, there set ups.
Robs point above is more serious, we have had shows stopped among other things because certain people of an animal movement think the whole process of keeping any animal in captivity is cruel, with amunition like some of these inverts feeding on vertibrates vids showing animals fighting (also illegal...called baiting) killing and eating one another in ways that can easily be proved as cruelty or unnessasary suffering (because the mantis, if hungry would eat the fish dead 100% of the time) eg mantis eating live mouse/snake/fish, tarantula stabbing live mouse over and over are causing an animal (vertibrate) unnessasary suffering, this could get some one banned from keeping animals but worse it could just be another nail in the coffin of herp/invert culturing etc etc