That fragile ego basically means narcissists are afraid of humiliation. On top that, mentally everything is build around shoring up my ego to compensate for the low self esteem. My ego. Not someone else's ego. I'd have to do some really weird mental gymnastics to be satisfied with copying someone else's work, that would just lower my ego even more. I guess it's doable if I straight up believe I'm that other person? But now we're talking about something really special and uncommon. Not just a narcissist anymore. The narcissistic self deception is involuntary, I can't just say "well, I know I'm copying the work of that person, but now I believe that's me." It doesn't work like that.
The person doing this clearly knew there was considerable risk of getting caught and being exposed. That's someone that doesn't give a fuck about the social consequences.
So you were closer with the malignants, but it's probably just a sociopath (ASPD). A lot more rare in general and more willing to take risks.