Like many things in trading, there is a sweet spot in the middle. Enough ego to be confident that you can succeed is required to become a good trader. Too much, and you cannot admit your mistakes (your mistakes are the main thing you learn from).
Narcissists can succeed by hiring the right people. Here is their modus operandi: hire an expert who knows how to do what the narcissist does not know, get it done, then the narcissist takes credit for it. The expert may not even care about getting the accolades if he is well compensated by the narcissist. A really smart expert will even try to make the narcissist think everything was truly his idea. Then the expert will get a bonus. Win-win.
Hey, maybe this little anecdote is a good diagnostic tool! Have you witnessed such a scenario at some point in your life? If not, maybe YOU are the narcissist.