Why Does It Hurt So Much If They Were Such A Jerk?

One of the "bad feelings" that come from separating from a person who has been abusive is the LOSS of an important relationship in our life.
Also, the loss of the people and the community that is connected to them, that we shared with them.
AND, if they have a Personality disorder like Narcissism, we know that the likelihood of them spreading false rumors about us is high to people they're connected to.
The likelihood of people who are connected to them also having personality disorders is also high~ (addicts connect with addicts when they're using; it's the same thing with pers. disorders)~
so they will often be very happy to join in "rejecting" us and spreading false rumors about us. The more people the person is connected to, the bigger the spread of the smear campaign may be. (Only people with Narcissism make "smear campaigns", but they almost always do it when they have some kind of "rift" or "falling out" with a person.)

ALSO, we know, if the person we're separated from does have Narcissism, they will probably IMMEDIATELY seek a "replacement" for us, whether we were their romantic partner, spouse, friend, or family member; even child or parent.
Once they find a "replacement", we know that the chance for reconciliation will be gone.
Narcissists REPLACE PEOPLE the way non-narcissists replace cars, or computers, or toasters. Once a "new one" is moved in, the "old one" is completely discarded and rejected, like the "old one" was some kind of "mistake", or "starter", or "cheap version". We know this, and it's painful to be rejected and discarded like a devalued object.

So that's a real "loss", even if the relationship was obviously destructive for us or them, and even when we really do wish them well and want them to be happy.

But REMEMBER, ONLY NARCISSISTS "discard" people that way, as if they're some kind of worn out object, or a child's toy that's been grown out of, or some kind of "pariah".
OTHER people who DON'T HAVE NARCISSISM DO NOT DO THIS, and they DO exist!