I have so much to say about this. First off:
BAD-FUCKING-ASS LAST WORDS
Arthur Blackthorn was a man who lived a very tourtured life, with a lot of pain. It’s easy to feel bad for him, and see this as a single moment of clarity where he saves the day, but I see it as a reflection of the cause of Arthur’s pain: Nerissa and Andrew’s relationship.
Andrew loved Nerissa, he had kids with her, and stayed with her for seven years. But when he found that Arthur was being tourtured, he turned against Nerissa, and everything she represented. He left her for dead - and he actually thought she was dead - without a second thought. Blood is thicker than water, and Blackthorns will place their family above their lovers. Arthur knows this, he had lived it before.
“I would rather be mad my way than yours”
Arthur was trapped by a Downworlder with supernatural powers. He was stuck, lacking freedom and bodily autonomy, and it drove him mad. Annabel’s torture by Malcolm may have been unintentional, but it was essentially the same as Arthur’s, leaving her stuck with her own - increasingly insane - thoughts. Arthur and Annabel are mad in the same way, they are victims, forced to watch and unable to act.
Annabel is not, “willing to do anything for love” crazy like Malcome is, (like Nerissa was) she is crazy like Arthur is, and Arthur knows that. He knows her mental state better than anyone else, and can guess what she will do. (She will follow his footsteps, ancestor mimicking descendant, kill the person responsible, the foolish lover.)
“I wonder if she will still love you, the way you are now.”
Arthur knows that Annabel will kill Malcolm, even if she still loves him. Because she will hate him even more, for treating her like nothing more than a puppet.
Malcolm thinks that he has the upper hand, which allows Arthur to get in several insults, seeing the oversights, and actually putting Malcolm off guard.
Arthur Blackthorn had a hard life, but in the end he was able to see the truth in it, and know what would happen next. He had the last word and he died triumphant.
Hail and farewell, Arthur Blackthorn