I ask the same question! No clue.

That's easy... Eva is a dentist. Clearly a torturer in her own right. Also is a supplier of needed sedation drugs and or gasses. Basically she was in on it too. But, since it was all handled the way it was, she slinked back into the shadows completely unnoticed, And thus had little bearing on the main story.

More important to ask, which it seems nobody does, is how does a CEO of a major corporation is under huge duress have time to stake out victims, kidnap them, secret them away to his torture chamber, feed them and torture them, and then do away with their bodies? Someone with that demanding of a job would simply not have time or the logistics to manage all that even without a side stalking, kidnapping, torturing and murdering hobby. Just the stalking alone would have taken up all his waking hours. And how does he get away with doing all that for years without his girlfriend and his housekeeper or anybody else ever catching wind of any of that? All the supplies and groceries coming in and out, the money spent, the trash created, the suspicious all hours coming and going from his secret door to the secret chamber that she never gets suspicious about or discovers. Totally implausible.