No deaths or violence involved.

lol, wtf? Is that a left over from being a kitten and getting carried around?

It is. Many cats, even in adulthood, will still go completely limp when they are held by their neck skin - like you said it's a remnant from their kittenhood.

That's not to say adult cats should be grabbed by their necks too often though, especially when they're heavy. Should always support them or if possible avoid doing it. But I can see how it could come in handy for professional animal handlers.

Yeah I've heard of this before, you should always discipline your cat apparently by grabbing it by the scruff and shaking it (gently) as that's how a mother tells a kitten she's displeased with its behaviour. Personally I wouldn't feel comfortable doing that, I wouldn't want to pick up anything bigger than a kitten that way and certainly wouldn't want to shake it.