Drugs and alcohol fueled spiral of self destruction... happens to a lot of people for no reason other than making bad choices.
I hope her spiral of self destruction hits warp speed.
Sometimes if serious mental illness is starting up they will try to self medicate without being conscious of it. Of course the street drugs and alcohol will just make their illness even worse. And the drugs and booze will mislead others as to what is really going on.
From what the ex husband describes about her behavior, including the sudden onset, it sounds a whole lot like schizophrenia. She’s probably just well enough to function normally most of the time but her thought processes are going to get more and more confused and her behavior will get increasingly bizarre. Killing the dog is example. It makes no sense and she probably couldn’t explain it if she tried. I suspect the boyfriend just went along with whatever she came up with.