I think JZB is beast too, for the same reasonsI'm a newby, so forgive my ignorance.... Here are my many hours of serious thoughts on the "Beast"....
Doesn't Mac refer to Barrons, numerous times, as having inhuman eyes? She states several times that looking into his eyes, she sees something other than human, a wildness.
The "beast" loves to kill, according to Ryoden. When Mac was inside Barrons' head, remember all of the gory, bloody scenes she sees? He kills man, woman, and child. I can't see him doing that in his human form. Maybe those are the beast's memories?
Also,in Darkfever, Mac says something about Barrons being 99% gone, 1% there (while they were with O'Bannion). Maybe he was focusing on his other form? The two probably unite to hunt.
Plus, the "beast" doesn't acutally start to die until she stabs him with the US spear. Isn't the spear deadly to FAE? Isn't Barrons a different race than human?
As for Mac feeling as if she lost her world, I think she was very distraught for assisting with killing the one "person" who continues to save her life. The book says the animal gives her a look that seems as if she betrayed it. She kills Barrons when he so often saves her. I'd feel betrayed! She has no way to get home, and the LM is after her- Barrons is dead. Who will keep her alive?
I hope I am wrong, but there are so many references to Barrons being beastly (not those exact words), and he is always stating some things are worse than dying. Maybe like being an infinite killing animal? Maybe that was what Fiona was going to say before he stabbed her. Barrons knew Mac was the one epic person who could kill him and put him out of his misery...