My mother is mentally ill. When I was 11, I realized that she was more than a little eccentric, she was severely delusional and dangerous. I spoke with my father and told him I was leaving, and asked him if he was going with me. He initiated divorce proceedings.
Not long after that, one evening she flew rather violently off the handle and started throwing my great grandmother's china at him. (No, of course she didn't throw the nigh unbreakable and worthless corelle, she had to throw her grandmother's platinum rimmed bone china...) He ran to a telephone (this was before mere mortals could afford cell phones) and started to call the police.
She ran to the basement. I had a sinking feeling I knew what she was up to, so I positioned myself on the landing. A moment later I saw the most frightening thing I have ever seen. My mother, with a look of rage and mania on her face, and a rifle in her hand. She was trying (and failing) to run up the stairs and load the gun at the same time. I screamed "she's got a gun" and my dad and I ran out of the house through opposite doors. We ended up huddled in the woods in the rain until the cops showed up.
People sometimes tell me I'm a remarkably calm person. The truth is, after you've seen your own mother with a look of insanity and rage on her face and a gun in her hand, about to try to murder your father and probably you too, everything else pales in comparison.
[Edit:] I haven't seen or spoken to her in over 20 years. The nightmares haven't gone away. I have dreams in which she's there - I don't even see her face, for some reason in my dreams I only see her hands, folded in her lap, but I know it's her, and I am filled with dread. Once or twice a year I even wake up screaming and thrashing and fling the covers off the bed and leap to my feet, ready to fight. Last time I pulled a muscle doing it and was in pain for weeks. I worry that if I ever find a husband I may accidentally hurt him if he sleeps in my bed. I wonder sometimes if I'll get any relief when she finally dies.