DARVO

There is a documented pattern of behavior by abusive people summed up by the acronym DARVO which stands for Deny, Attack, Reverse Victim and Offender. The Internet is rife with people rampantly using the term narcissist for all kinds of abusive behavior and I once told someone on Metafilter "What you're describing isn't narcissism. It's sadism."

Narcissists are selfish people who couldn't care less what goes on with you and they shaft people if you are the responsible "my word is my bond" type and imagine they are too because the minute they have what they wanted from you, they forget all about any promises they made to get it. 

Narcissists aren't that hard to deal with: Get YOURS FIRST. If they can't keep their promises, no more "credit" gets extended. Demand cash upfront always for everything or don't do business with them.

If they are friends or relatives, in addition to getting "cash upfront," you can give as much as you feel you can comfortably spare without expecting anything in return as long as you are clear you can spare it, it's your choice and you will just say no to other stuff.

Do NOT be similarly generous to selfish, demanding jerks that aren't already some manner of part of your social circle because it will be liberally and intentionally misinterpreted as setting a precedent like you signed a new social contract in blood to be their bitch forever and ever because you did them a favor once.

That doesn't mean don't ever be kind to anyone. 

Narcissists mostly hurt you unintentionally. They're just selfish people with often various personal problems, like they sometimes don't really remember what they said when they made that agreement. They're just trying to get their needs met and they are kind of oblivious to social stuff and tend to be oblivious to the fact that not keeping their word can bite them in the butt later.

Once you know you have a friend or relative or coworker who is like that, just don't take their word for anything because their word is no good. You don't really need to psychoanalyze why or try to somehow help them do better.

If they are legal adults and not your child, their inability to get their act together is not your problem. In fact, it's typically dysfunctional on your part to feel compelled to get up in their business and then your crap and their crap tend to interact badly and go boom and it gets difficult to sort out who is "at fault" once that gets started.

If you aren't, say, marrying them, find a better method to interact or walk away. Even if you are literally marrying them, there are limits to how much you have a right to impose your expectations unilaterally on their behavior.

Your problem is interacting with them without being shafted by them all day everyday in every interaction. So just stop doing stuff for them where you feel imposed upon and like it's excessive.

Learn to say no and make it stick. If they won't take no for an answer, start moving them out of your life. 

In most cases, this is the beginning of the end. Most people are creatures of habit and narcissists are socially challenged anyway, so typically it means you just gradually move on from these people.

Once in a while, one of them will want to keep you in their life and will decide to figure out how to do that and they may ask what you want or they may analyze things and stop their garbage.

Unlike your typical so-called narcissist, sadists aren't merely oblivious and selfish. They do what they do because hurting you is the goal. And I knew the person on Metafilter was talking about a sadist because the person in question told them at some point that they knew beforehand that "helping" them in that way was actually shafting them.

Sadists not only want to hurt you, they typically want you to know they intentionally hurt you. There's no means whatsoever to have a healthy, constructive relationship with someone like that who hurts people for fun, without provocation etc. 

This is distinct from someone who has baggage and issues and pursues schadenfreude or some kind of emotional outlet via "dirty jobs" society needs done. Both types of people get called sadists and I don't really have a good way to distinguish the two other than DND alignment terms chaotic evil and lawful evil which I don't find satisfying because this second type of person isn't actually evil. The first type absolutely is.

Abusive chaotic evil types engage in DARVO and typically want to act like hurt little children and insist only their feelings matter and nothing else matters and insist they have a RIGHT to hurt people because someone else hurt them first a long time ago. 

They demand endless second chances. They have ENDLESS excuses for why no one can EVER hold them responsible and make ENDLESS empty promises that "This will be the last time, I promise!" and similar.

And they COUNT on an ever-changing social landscape of a new school teacher every year and a new judge in every court case and a new boss every single time they change jobs to let them blatantly LIE their asses off and get away with it.

If society or the courts buy this BULLSHIT and never check their story or give them another second chance even though they have some kind of known track record and everyone knows it because they are famous and you have read about their garbage in the newspapers, welp, don't be shocked when their crimes get ever more outrageous while they feel increasingly confident nothing will stop them.

If you are a judge or other social gatekeeper of appropriate behavior, please seek affirmative evidence that this time for realzy realz honest and for true is the last time. Or throw the book at them if they can't manage to supply you with such.