Whenever a narcissistic person feels threatened, they will call you the things that they themselves are as or are afraid that others see them as. And then they will try to stalk you, slander you, or discredit you. They will try to sabotage and destroy you. They will start a smear campaign and attempt character assassination. In their mind, frighteningly, you have become their mortal enemy.
They also have no problem doing all of it preemptively and calling it defense.
So if you privately call them out, set healthier boundaries, or end the relationship, they may be afraid that you can see their flaws, or that you will tell others what kind of person they are. Whether you do that or not is not important to them. Because in their mind the mere possibility of it is a good enough excuse to label you as an enemy. And because a narcissistic type of person has little or no empathy, they may imagine that you will behave as they would in these situations. If they would lie, or more likely are already lying, they will accuse you of lying.
And so they will do all these things just because they think you are somehow trying to or might hurt them. They also will accuse you of the very things they themselves are doing.