Narcissists are absolutely a thing, but it sounds like you're conflating two groups.
The poly relationships I know, and the open relationships I know, just aren't even close to what you're describing. Even the closest singular example to what you're describing — one out of a dozen people who are open about being poly and a dozen more who are open about being open — was, despite being extremely selfish, not sadistic as the person you describe appears to be.
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Narcissists are absolutely a thing, but it sounds like you're conflating two groups.
The poly relationships I know, and the open relationships I know, just aren't even close to what you're describing. Even the closest singular example to what you're describing — one out of a dozen people who are open about being poly and a dozen more who are open about being open — was, despite being extremely selfish, not sadistic as the person you describe appears to be.
If they are open with it, that is definitely not narcissism I agree.
If they hide it, plus express all the other dark triad traits (sadistic / malignant behavior) I agree its different.