[user104658]

As an "excuse"? Difficult question really. Mental disorders will often genuinely make people mistreat others... it's not necessarily an excuse, as in, it's not that they just "wanted to do it" and are falsely claiming that they can't help it. Often, they can't.

On the other hand, that's different from saying that people have to accept hurt and mistreatment just because the person dishing it out has a mental illness or personality disorder. Understanding and accepting the reasons that a person is the way they are, even forgiving them for it, is a good thing... but that doesn't mean you have to keep that person in your life, if they are having a negative impact.

Also there's a world of difference between someone who knows that they have issues that are causing them to mistreat those around them, and when they have more clarity are genuinely sorry for their actions... and someone who thinks that they "don't need to be sorry because they can't help it".