The behavioral variability in narcissism and psychopathy is only apparent. Actually, their behaviors are very consistent and predictable.
RULES
Optimizes allocation of resources, resents when forced to act wastefully and becomes aggressive, rejecting, frustrating, and sadistic.
Do you care about me? Narcissist: I do. You are useful to me. But, don’t you have emotions for me? N: I don’t do emotions. I do relationship maintenance. I do business: give and take. So, why stay in touch? N: I owe you. I repay my debts. Plus, you could still be useful.
Example:
If s/he wants to have sex, s/he will turn on superficial charm, flirt, lovebomb (idealize).
But with the same subject, when s/he cannot have sex (even though s/he may fervently wish it) s/he will be indifferent, dismissive, bored, and impatient.
So, motivated strictly by opportunities (opportunists), not by people, emotions, cognition, or social mores. Loses all interest when no opportunity presents itself.
Similarly, will disengage physically and mentally and move on once task accomplished. Will not linger or engage in social niceties or small talk.
Is utterly uninterested in other people only in their functions and possible contributions. They cease to exist once their utility is over, when no longer useful, and are dropped abruptly. Everyone without exception is instrumentalized and objectified.