The narcissist cares only for one thing: themself. To the narcissist, it's all about how they can gain from us whatever it is that they've determined that we can provide while investing as little as possible in return. We're not coequal persons cooperating with them in building a friendship based on mutual trust and respect; rather, a narcissist sees us only as objects to be used in achieving their goals, filling whatever needs—emotional, material or otherwise—they may want from us based on the roles in which they have cast us.
Seeing us as objects, as a means to their ends in their search to extract the maximum amount of personal benefit and pleasure from this life regardless of the cost to others, the narcissist doesn't even conscience that there is a cost to their victims, that they're hurting another person, because they've so dehumanized us that we're nothing more than beasts of burden and steaks on the table to them. We're but a commodity for which there is a limit to how much they will "give" in return for our output, and they'll cut their loses and cut us off without a moment's hestation when that time comes.
We should learn early on that it's a waste of time to speak of truth or facts. Insist on the truth, and they'll give us some line about "their truth", how that they aren't bound by "our truth". Any facts that don't line up with their narrative are obviously "alternative facts". And the moment "proof" is offered, they'll cut us off and execute their exit strategy, doing all that they can preemptively to destroy us before the facts and truth show them to be the sociopath that they are.
The moment that the narcissist decides that we're no longer an easy supply, that their use of us requires more energy than any of the other targets that they've lined up as our eventual replacement, they'll move on and replace us without any hestation. If they realize that we're done being used as their supply, that we're regaining a sense of our own individuality and worth apart from them, they'll cast us away, discarding us without a second thought. If they think that we're realizing their true nature, especially if we might be able to prove them to be the vampire that they are, they'll attempt to destroy us, running all manner of slanders though the gossip mill, convincing their minions that we're the evil one, that we're the greatest threat to their peace and happiness.
Quite frequently, we'll find that the narcissist preemptively has laid the foundation for their ultimate betrayal and abandonment of us long before they actually executed their final plan. They'll cry about how we used them, hurt them, left them and abandoned them while they were always there for us, or tried to be but that we wouldn't let them. They'll try to convince their new targets and minions that we're the one to blame for any and all problems. They may claim that they wanted to work things out but that we just wouldn't work with them. They may go so far as to accuse us of being the abusive narcissist, if not even having committed acts of varying degrees of moral turpitude. They'll paint a portrait of us that reflects everything that they did to us so as to poison the water should we tell others what they did, and the tales will be so horribly grotesque that their targets and minions will believe us to be the inhuman monster that the narcissist actually is.
In all of this, one of the hardest things with which to come to terms is the fact that it was all a lie. Everything that the narcissist ever said and did was an elaborate rouse to draw us into their web of manipulation. They never loved us. They never wanted the best for us. They never were a true friend to us. They have no empathy, no moral compass, no sense of loyalty, duty or compassion. They aren't like us. They can't consider that there is an objective standard of mutual comity and reciprocity in interpersonal relationships that directly applies to them, that they should have a desire to return in kind the kindness and considerations shown to them.
So what do we do? I know what I do. I still love them anyway. I forgive them. And I walk away, leaving them with the responsibility to come with a sincere apology coupled with a willingness to be accountable if and when they should ever self-realize and get the help that they need, knowing that far more often than not, they'll never be back if we consistently refuse to play their games.