trying to adopt an amoral worldview

[John 6IX Breezy]

Most girls do like nice guys.  They also like confidence and success.  

My own personal take on this is to strive for competence not confidence. There is of course an internal value to feeling confident. However, if there's one thing I've gleaned from the psychological literature its that confidence rarely maps unto reality. It tends to not have predictive value. People report feeling confident about a great many things that they do poorly on; even to the extend of outright delusion. So I've come to the conclusion that confidence has very little utility beyond mental health and possibly attracting a partner. And I'm not entirely convinced it does even that reliably well. Your display of confidence can be interpreted as arrogance by one person, and cockiness by another, both of which lowers attraction.

Competence in contrast has utility. It is by definition the ability to do things well. And I think striving to do things well creates the kinds of growth than will reliably be seen as confidence by others. And it opens the door to the kinds of successes that will be considered attractive. 

As for the niceness problem. I think the solution is to strive to be a good man not a nice guy.