>>96052942
Well, assuming it's a recruiting cult, which most pre-modern ones aren't (you were not converted into sacrificing people on top of pyramids, that's just the culture you were born in).
But assuming it is, there's two main mechanisms, I think:
>Escalation of commitment and psychological entrapment
You do something a little nasty as an initiation ritual, then you do progressively more and more fucked up things, because looking back and realising how far you have gone would be too psychologically damaging.
It's a great way to keep the group tightly knit together.
See: whatever they are doing at the Bohemian grove
>Looking for a sense of belonging
If you feel outcast, basically anyone coming to you with an open hand, saying "come, brother, we are in this together" will fill the need for a community. Then after that, their moral will become your moral and if you don't have anyone in your life to bring you a different moral perspective, you wouldn't know any better. Even better, the gap in moral you will have compared to the normies will add to your sense of superiority over them and increase your sense of fraternity with the cult.
See: ISIS, or skinhead groups
Then, if the group gains a bit of traction, that's when you get the normies joining so they aren't left out. Then, the nasty stuff is directed at scapegoats and/or a way to show your commitment, both of which also increases group cohesion and fanaticism
See: the Chinese cultural revolution, the SJW, the anabaptists...

There's plenty of IRL examples to choose from, what I find surprising is that you find any of this surprising, really. I mean just look at /pol/ and ask yourself how you can get people openly claiming that they long for the killing of a sizeable proportion of the population.
I'm no specialist though, so I'm open to hearing that I'm wrong