Well, it's a speculation of course but I assume they follow a sort of satanism invented by Sabbatai Sevi a rabbi of the 1600's where they get a "redemption from sin" by doing another sin. (My sense is it needs to be a worse sin) There might be a certain strict format in which they do it, like you say: maybe they need some sort of consent. Like they might invite someone to join them in a ritual and tell them that they will be the center of everyone's attention there but leave out the fact that the reason they will be the center of attention will be because they will be ritually slaughtered.

Their techniques might actually delay the karma laws while they are alive - but not after that. Since they don't talk to anyone in the after life they don't know & so this sick cabal has gone on for centuries. My sense, though, is that they need to do more & more heinous sins just to keep this going. If they are already up to the sin of genociding the entire human race, I think they are about maxed out & so, my guess is that their power now will start to wane.