I think it's more accurate to describe Dark Danny as an evil version of Danny or a dark copy. Danny was human first and the ghost half came later. So the human Danny was the real danny and the ghost half was more of a shadow with all of Danny's memories but without his heart. You also can't really say that Dark Danny is evil Danny but rather Danny's dark side given life without concience or soul. In order for their to be a true evil version of Danny, Danny has to willingly turn his back on morality and walk the path of darkness of his own free will. Dark Danny has nothing human in him so he literally has no agency and can never not be evil. I also think that a Danny Phantom that chooses the path of evil would be worse than Dark Danny as his evil would be drawn from pain and sorrow. After all, how much you are capable of hate depends on how much you care about something in the first place.