I'm glad you replied, I was worried that I came across in a way I didn't mean to. . . but the more you elaborate, the more familiar that situation sounds. . . now I feel so bad about even questioning the toothbrush thing. I'm about to share a similar story, however, it doesn't hold a key to someone going through my personal things. . .
You're a saint, and an amazing roommate/ overall human. I have also lived with a close friend (she moved into my house for almost no rent, as I wanted her to be able to save, feel safe and know that she has people on her team, after some crappy life circumstances) . For the most part living together was A-Okay, and I was very forgiving because I kinda knew she had no one to vent to, or cared much about her (I freakin' did! I still do! I really hope she works through it all)
I'm seeing many, many parallels here, too. One of those people that BOTH sweep away legitimate concerns of others, yet will cause a scene if they actually have to face the realization that their actions negatively affect another person, even in the most basic ways. Granted, I wouldn't toothbrush-toilet water her, because I feel like if I did it would only have made her worse. . .
While she didn't abusively harm my dogs. . .we have a protocol with our dogs that's like "they are our babies, please lock the doors and clean off the counters, so the big dog isn't tempted to jump up." SO, after multiple times where she did not lock our doors, and would laugh it off . . . but then throw FITS about anyone having the audacity to offer any feedback to her. I don't think she ever actively hurt our dogs, there was definitely the lack of "being a human being" in many ways. I was always so damn sweet and understanding, and more of a family member than her abusive, neglectful addict parents ever were . . . but she had ONE JOB! To shut the door, and to keep my puppers out of harm's way.
Aside from the dogs, she was aware that I've been robbed before, while asleep, and the burglars came into my house and looted it, sooo like. . .please lock the doors. LOCK THE DOORS, DON'T LET THE DOGS OUT WITH UNLOCKED DOORS.
At this time I was living in the Southwest, and the temperature is dangerous and deadly to leave dogs outside longer than a few moments. . . but I came home to find the door to my home not only unlocked, but she had actually left it swinging wide open (after many reminders, I even wrote her a haiku about remembering to lock the door). Then, when I walked in, my two little dogs were left outside not only with the front door open, the french-door to the yard was swinging wide open (both deadly hazard to the little dogs and very pricey). When I confronted the situation, I was amenable to listening to her and, ya know. . . if you leave someone's house open with their dog-family loose, apologize. She did not. She said horrible things and was furious that I was communicatively upset, but not nasty. . . simply said "It is NOT OKAY to act this way, to leave someone's home at risk, and leave the dogs vulnerable."
I thought it was pretty concerning that she left my dogs in harm's way, but I hate to ask, but am also curious how she harmed a puppy? That's the ultimate "I am worse than a movie bad guy" move . .and makes me think she could benefit from a really good in-patient program for people without empathy/conscious.