I see the term narcissist and variations on it (narcissistic, narcissism) thrown about a lot on the Internet. It seems to be used to cover a wide variety of asshole behavior.
And I think it gets used sloppily to cover things that really are something else.
In a nutshell, narcissists are what my mother would call selfish people. They care only about themselves and no one else.
They are relatively easy to deal with compared to other forms of psychological dysfunction. If you know you are dealing with someone like this: Get yours FIRST.
If they whine and cry to you about how desperately they need X, tell them they can have X after and only AFTER you first get Y.
Do not budge because they typically do not keep up their end of the bargain once they've gotten theirs. Hold their thing hostage until you get what you want.
If you don't, you are probably doing stuff for them for free. Probably AGAIN.
I once saw a question on Metafilter from someone saying her father was a narcissist and detailing how much harm he had done her. Then it came out in comments he had INTENTIONALLY shafted her financially and made sure she knew he intentionally shafted her.
I told her "He's not a narcissist. He's a sadist. Get out."
Narcissists think ONLY of themselves and not about you at all. No, it doesn't bother them if you get shafted. They don't care about you.
But they probably aren't intentionally shafting you. They just stopped being invested once they got theirs.
In contrast, sadists are very invested in hurting you. Hurting you is the point and they typically go out of their way to make sure you know they hurt you.
It's not any fun if they can't bask in the misery they caused.
If you do X for someone on the expectation they will do Y and you don't ever get Y, it's possible they are a narcissist. That's par for the course for such people.
But if they aren't merely lazily failing to "pay up" and are instead going out of their way to cause you harm and make sure you know it, it's probably a sadist.
I generally recommend doing all in your power to move sadists out of your life. It's extremely rare for them to be trustworthy, though there are exceptions.