You're half right about the last one, they just showed and taught the humans about adrenochrome as a way of encouraging a certain behavior(turning souls self-serving, it all about the souls and about them bolstering their ranks)..

That said, the vampire myth origin more than likely stems from people drinking adrenochrome, most vampire myths paint vampires as noble/royalty or in some way from or connected to some upper echelon... which would be exactly the demographic drinking adrenochrome...

It's not for nothing that areas of the world that have/had long lasting and established cultures/societies are also the areas with some kind of local version of the vampire/blood-drinker myth.....

It the same exact modus operandi, century after century and in every society. Infiltrate, encourage behavior, reward it by only letting the worst rise to the top, continue until everyone in charge are theirs, be it by breeding(royal houses of the world) or corruption and leverage.

Whatever they are currently called(kazahrians, DS, whatever), it's the same continuous 'club/religion' and they have been going for several millennia....

There are many players involved, not just the pointy-headed lizzardy types and there are many factors involved, the adrenochrome thing is just a factor in our neck of the galactic woods, it is an ongoing low-intensity 'war' that has been going on for as long as souls have existed, there are good players and there are bad, just like with criminals in any grouping the bad are a minority, they will never win but they will never go away either so keeping them in check/fighting them will never stop either..