AI from beyond the blackwall are trying to inhabit human bodies, thats whats going on with the bloody ritual cyberpsycho sighting and a few other small lore entries in the game. Erebus and Canto aren't soulkilling people at all, instead they're opening gateways so that rogue AI can take over their bodies. Unfortunately for the AI / fortunately for everyone else they're unable to do so succesfully and end up burning out the brains and cyberware of the host almost immediately. That scream is basically the victim undergoing Vs relic problem of being taken over by another personality but with the entire run of the 'illness' condensed into a couple of seconds.