Not to excuse anything, but the context of any comments must be taken into full account.
When a BS finds out that they are not the apple of your eye, and that they are a BS instead, those who are the most trusting and felt the most secure, can be so devastated that they are not actually operating in their "right mind". I know that I wasn't after D-Day, I wasn't able to sleep for 3 days and I still had to go to work (self employed).
I would equate the N- word with a lot of other words that can come out in that state of rage and despair (I've heard that word and other racially charged words used in my lily white redneck upbringing and as a small child used some of those words till I was taught better by my parents).
No, I don't condone it, nor do I use it, but I still say words that my parents would never condone (in other words, I curse rather freely and casually without a lot of emphasis, and take the name of God in vain as we would refer to it in the church I grew up in, I curse more in a single day than I heard my parents combined use in my entire 18 years growing up). Curse words are just words to me, I use them casually.
But, on the other hand, I'm pretty tightly controlled overall, but I still don't know how I was able to keep my grip on my emotions to keep my cursing in check when my wife confessed. It was indescribable. I usually curse about simple things, but there was absolutely nothing I could say that would express my emotions, cursing just doesn't carry that kind of "oomph" to me, perhaps that was why I couldn't even bring myself for the most part to rip off a string of obscene commentary like I would when a project is not going well.
However, I would never use the N- word, or the C- word for Chinese or Asian persons, or any of the other words that I grew up hearing.
I suppose, if you put me in the same situation as your BS, because that N- word would be such a violent transgression of my norms, that I might have done the same thing, because nothing else would convey the rage and despair adequately. I don't know though, and don't want to find out.
FWIW, I knew two people who apparently NEVER used the N- word, until they were ill (but they were raised in a frankly racist society construct), were avid advocates of equality, one was a freedom marcher and civil rights activist that was quite well known where she came from. When ill, they were not "in their right mind" and both went on to die, and in that phase they both said terribly racist things that mortified the people working with them, caring for them, and their families. One was a man, and his wife of many years said she couldn't believe the things he said in the hospital. The other was a woman, and she was even worse, and directed her comments toward medical staff who were not white.
So, perhaps to be more brief, I would reason that knowing we all have latent degrees of racism inherent in our upbringing, we should be place it in the same place as other forms of verbal abuse, misogynistic, racist, or misandristic statements, etc. NOT OK, but not really surprising if taken in context.
Your brain is a sponge of information, we all have shit like this in there ready to pop out when we are not functioning well.