Suzy, that's so disturbing that you read the things your mother wrote when she was feeling at her worst.
I agree that there are some things are best being torn up and discarded. I have thrown out pages from my own journal because they are just nasty and negative. I wouldn't want anyone to read them... because I don't mean half of what I'm saying when I'm angry and writing.
My sister found her diary from her teen years when we were both in our 20s. We sat down to read it together and it was just full of her ranting about how much she hated me. We sort of laughed about it while reading it and she told me she didn't feel that way anymore; she was just a messed up teenager. It was hard for me to know that my sister had hated me so much, though. I'm 5 years younger than her and looked up to her and adored her when she was a teenager. I had no idea that she felt such animosity towards me at that time.