Oh man, where do I even begin

I had a friend who I met from another friend - we'll call these two A and B. A told me that he had a crush on B and that he was in love with her - keep in mind that they had only known each other for maybe a few months. Anyway I think that's neat and don't think much of it until I start noticing how he lacks any self awareness about his actions - he begins becoming quite clingy when talking to B and also starts saying some things that are quite inappropriate without realising it like when he recounted an incident where she "straddled" him (for reference, she was trying to get her phone back since he had it).

Anyway so B decides to stop texting A but still he persists - continuing to text her, sending her an occasional :( when she doesn't respond and making an effort to continue asking her to hang out. At this point he becomes quite sad and starts talking to me about what he should do - I try and help as best as I can whilst also helping him with some other problems he had going on, he would often come to talk to me about things that were not so good in his life. He said he thought about B everyday and became really sad that she was no longer talking to him - I knew why she had stopped but for the sake of my friend, I didn't tell him and tried to comfort him as best as I could.

Anyway fast forward maybe 2 or 3 months and there's a day where he snaps - I don't really know how to describe it but he sent a long series of texts talking about how he felt quite emotional and dark and that he would hunt me down if I ever stopped talking to him. He later said he had been in a bad mood but that really put me off so I made the decision to stop talking to him - I think after a few days he asked if I was ignoring him and then left it. The next day I'm hanging out with another friend and some guy I hadn't met before - I tell them what happened because honestly it was all so ??? and the guy I don't know asks if he wants me to speak to A. I say no as I want to defuse the situation and leave it BUT THEN he starts a full on fight over Messenger, starts hurling insults and telling A to stop harassing - A becomes angry and the whole thing is a very big mess.

So now it's more uncomfortable though I do my best to apologise and move on from the incident- anyway nothing happens for a while and then B, suddenly gets a letter. From A. It was like a 4 - 5 page letter - there was no apology just asking her to talk to him again and how he saw her as a hero - it also had a gift inside though I can't remmebe what it was. But like, YIKES. The whole thing was wild. I often feel bad thinking about how I stopped talking to A but I felt like there was just something emotionally toxic which both B and I agreed on so we cut that friendship off.