Narcissism (along with other personality disorders) is simply the consequence of a stunted childhood development, which is all but certain to occur in those with autism.
Elliot Rodger was no exception, and he coped with a confusing and threatening reality through narcissistic defenses.
The difference between him and Bad Boy Brads, who also tend to be narcissistic, is that he had to fully face the dissonance between his grandiose delusions and his pathetic reality, and this resulted in a fatal episode of mortification and rage.
He was not exactly psychotic, and his distorted thinking is fully explicable by examining his autism and co-morbid narcissism (and besides this it must be said that his aberrant nature gave him many kernels of great insight and made him an exceptional writer).
I sense that you oppose the description of Elliot Rodger as a
narcissist because it has negative connotations (even beyond those of
psychotic)
Narcissism is not a euphemism for evil or anathema, although I am guilty for using it in this sense myself.
The fact that Elliot Rodger was peacocking in a prosocial way is actually typical of narcissists. They affect the image that they
think will make them liked and desired by other people by consulting their
superego (the standards of conduct that the prevailing culture has inculcated them with).