I'm 37 and In my life time I've have experience of 2 very toxic friends. I am the type of person to never rock the boat or fall out with ppl. I will always try to keep the peace.
What struck me was your description ...self centred.
Friend no 1 (I was about 26) became friends but soon realised she was only friends with single girls and would dump you if you had a boyfriend, she was incredibly selfish and intimidating. She was drunk, I refused to be her wingman because my friend had turned up and I wanted to say hello to him. Ex friend then threatened me that if I did that she'd tell his girlfriend that I was trying to come on to him

it was very hurtful and seeing as he was an old friend and my ex friend didn't even know his girlfriend it was laughable
She then tried to argue with me (I was sober) and got in her car and drunk drove home. No one ever heard from her again.
Friend no 2 ...all through uni and into my 30s she was my best friend. I defended her a lot because many ppl called her spoilt and a princess. I thought she was misunderstood. But, looking back she was very rude to ppl.
She saw a post on Facebook from a friend's boyfriend who apparently said something racist. I fully supported my friend and listened to her, was totally there for her. However ...and I'll never ever know what triggered it but days later she
She sent me a vile essay type email saying I was racist, that she was a strong woman who would raise Muslim boys, that I was scum. It was shocking.
For context I work in international business and work closely with the governments of Pakistan, India and the middle East. I was also a religious studies teacher for 10 years teaching about culture, religions, prejudice and discrimination....whilst she hadn't done anything like that....so at least accuse me of something plausible.
The letter was vile, picked me apart in every way. The content didn't ask for my opinion or ask me questions or explain why she had sent me the letter so I never responded.
About 2 years after the incident I was at a wedding with lots of mutual friends.
I then found out that said ex friend had emailed all of them, individually to say I was a racist, nasty person. Some ppl who got the email didn't even know me very well!
Not one person believed ex friend but it was hurtful and I was really upset.
Since the moment she had sent that initial nasty email I knew she must be very very unhappy in her own life. I did find out her engagement was broken off a few months after.
My advice ....do not give this person an inch, they are not your friend. If you try and reason or explain she will use this against her.
In my opinion just don't respond.
Draw a line in the sand and say from this moment on you won't be engaging ....do not tell her, do not confront her.
If she continues to message you say you unfortunately will be ending the friendship and block her.