To start, I'll define what I view "dark traits" to mean personally to me. They mean having an internal moral system where it is acceptable to harm others in some ways.

The more harm a person is willing to do, the more we generally label those people with the personality traits that we say are "dark".

There is some measure of psychology that would indicate that many such people who we label that way have neuro-atypical thought patterns. They may even simply feel differently about things.

I personally view freewill / personal decision as the primary factor in harming/not-harming others though.

A single person can be all of the following simultaneously:
1. Empathetic ( able to understand what others feel and also feel those emotions themselves )
2. Willing to harm others ( disregarding how they feel )
3. Successful.
4. Appear to be neurotypical ( for the most part )
5. Be "team players"
6. Be narcissistic
7. Be manipulative
8. Be happy

The false conclusion made by most is that harming others is incompatible with the other modes of thinking. That is unreasonable as it is relying on "holistic stability of the mind". There is no guarantee that the thought process or emotions of a human being are internally consistent.

There is also no guarantee that the observable actions of an individual reveal their internal processes.

Since this is a throwaway, I'll take this a step further. I'm okay with actively harming others, either for my own amusement or my own gain.

I can understand how those I would harm feel about it, and understand how terrible that feeling is. It doesn't prevent me from causing this feeling in others. I simply choose to do so. I may occasionally become overly fixated on the emotion, but I am capable of dismissing it.

I am very successful and command a high salary. I am well liked by my peers. I am commended regularly for my work. I am considered a "team player" and am regularly asked to mentor others ( and do so as well ).

Those around me believe and say that they think I am neurotypical even when I have confronted a select few that I am not.

I morally believe that taking advantage of others is "wrong", but I still am capable of doing it, and I do.

I'm happy about who I am. While I do occasionally ponder the morality of my actions, and feel what you might call "guilt" over them, I choose to actively dismiss these emotions and carry on anyway.

I don't think I'm the only person who is this way. In fact, I'd wager that the majority of people are capable of harming others for their own gains, and able to dismiss guilt over it given a bit of effort.

I do agree that I am atypical in the amount of harm I could do and easily dismiss it... but I don't think any of these studies are able to easily "detect" me.

Put simply: if there exist light/dark people, I'm definitely on the dark side, and it absolutely enables me to succeed in ways those on the light side could not.

I will though meter this statement a bit by saying that being on the dark side also can be constraining. There is less mental effort needed to simply choose a solid moral base and stick to it. Most of the atrocities in history have come from highly religious groups with strict principles. Blinding believing in some set of morals is very enabling.

So it is inaccurate to say that being dark/light will cause success either way. There is simply too much variance and complexity with the ways a human can approach life.