manipulative/toxic best friend possibly suicidal, don’t know what to do

(this post is way too long, so I put a tl;dr at the bottom. I tend to say too much) I've had this best friend for about a year and a few months now. They helped me during my lowest point, even though they didn't know it. So I felt like I HAD to give back to them. Nine months ago is when they started to decline. Originally, it was over something they could not control. I really loved this person, so I did everything in my power to help them. But then they would do mean things and even embarass me in front of others, and never apologized. NEVER. For anything. If I told them their actions upset me, they'd get very mad, ghost me for a few days, then come back like nothing happened. One time, they ghosted me for an entire month after a small disagreement. They never explained nor apologized. Eventually, it just grew toxic, even if I didn't want to accept that just yet. I just wanted to believe my closest friend wouldn't do manipulative things. They started guilt tripping me about things. They'd freak out over anything. They have severe depression at that point. One time they put themselves down so hardly because they made a typo (one letter off) so I told them they were overreacting and needed to calm down, then they started saying I was calling them crazy and shit. Then I told them off because they were being very dude. They then ghosted me, for longer than usual. And when they came back, they'd said something about how my remark hurt them oh so much. And I apologized! I could never stop worrying about them. Every day I was worried they were going to do something to themselves. They had once before. I have OCD, so I REALLY couldn't let it go. Eventually it just got too much, watching who I once loved with all my heart destroy themselves when they had options for support (their parents are super supportive and they could easily go to therapy). I just felt used and I was panicking too much about them everyday that I eventually cracked. I sent them a very straight forward and calm message, saying that I will always be there for them, but they can't come back and talk to me until they get help. They seemed to understand very well and we wished eachother goodbye, but that didn't last long. They blocked me on some stuff and deleted their instagram account. How typical, I should've expected that. I have a deep hatred for the emotional scar they have given me. I can't support anyone else anymore. I'm so tired I just can't. I don't want to talk to them anymore. If I'm being honest with myself, I think part of me truly hates them. But I don't care anymore, because I'm so pissed. The issue is, they may be suicidal, because they have expressed some of those thoughts before. My OCD has probably gotten in the way of my thinking again. I could send a letter to their parents telling them they need help (since they refuse to ask for any. they also live very close to me). The thing is, I don't even want to. I really fucking don't. I don't know if I'm an asshole for saying that, but it's true. It's just that, if they did end up comitting windpipe-no-more, I'd feel so guilty. I couldn't live with myself. I don't know what I would do, knowing I could have told them. If I'm being honest.. the main and probably only reason I want to send that letter is so that I wouldnt feel guilty if they did pass away. It would still hurt a lot, but I would be able to accept it for what it was. I've given them so many chances to get help, but they refused and manipulated me, so why should I care anymore? The thing is, I know they want me to reach out to them any go "I'm so sorry 😭😭" but I won't fall for that again. My question is: Am I a monster for only really wanting to send that letter just to get the guilt off my concious? Should I send the letter to their parents, or is it my OCD and irrationality speaking? Are they actually at risk? Would someone at risk for out of their way to be so petty and mean? I turned 15 two days ago, so I want to get this over with so I can spend my teenage years in peace. TL;DR, very toxic/manipulative friend may be at risk of hurting themselves, but that may just be them manipulating me again