Narcissists can’t accept that they may not be wonderful people. They are also incredibly fragile when facing an idea that perhaps they did something wrong, especially if others can see it. Therefore if there’s a conflict they will do anything and everything to maintain a fantasy that they are always good, all while perceiving the other party as evil.
Not only that, they need other people’s validation that their delusion is true. To achieve that, they create preposterous, slanderous, manipulative narratives where all of that is true and try to convince others of it. And since many people are unwilling and unable to look into the truth behind it, the narcissist can find that validation they so desperately crave and even act out their revenge fantasies. Often the reason is as simple as hating to see others doing well because they themselves are miserable.
As a result, sometimes people get seriously hurt: socially, financially, emotionally, or even physically. But the narcissist doesn’t care about that. In fact they are often glad, because in their narrative the victim deserves it by being “evil,” so whatever happens is justified.