[Serious] Redditors who have completely ruined somebody's life (intentionally or by accident, whether they deserved it or not), what happened and why did you do it ?

Well you see, since Middle school started I had it all. I met someone who was actually really nice who later became my best friend. 7th grade came and it was fucking awful but I made a new circle of friends and two of them were always a little "off". This kid named "Matthew" had a repetition to be "bullied alot" but he just got offended by fucking everything even calling him "Matt". Then there is this columbine style kid named "Mark". He seemingly complained about his family alot but now you could see why because he was fucking deranged. He would pick fights with people he knew long time friends if they did one thing wrong, but mostly nothing or all. Also, he would round up his little hate group to not talk to them or just do stupid shit. I was friends with Matt at this point but he didn't like him talking to me and would threaten to stop being friends with him and at that point I did care. Well since then, even with him being an asshole we kind of became friends but their little relationship was really annoying my close friend Aaliyah. So time goes by a little bit and I'm in this stupid fucking group chat of theirs that I stopped being active in. Sitting around in class just having a good fucking tie and he decides to start a bunch of shit by threatening to kick me out of the chat and I really didn't care. For some reason this makes him mad so he thought he would just troll me for the rest of the day and I wasn't in the mood and successfully ignored him. As things go on to the next day he starts ignoring me and being a fucking asshat for no reason but I didn't want to be friends with him he was whiny and unbearable and it felt like a huge weight was lifted off my shoulders. Now to the ruining his life part, he had threatened to shoot up the school and even mentioning columbine and that he was "gonna kill EVERYONE". He even walked out of class shortly before the bell rang because he didn't want to "DEAL WITH THESE PEOPLE". Anyways back to the main story, in reteach he is passively aggressively saying shit about me and it's obvious because fuckboy matthew pointed at me once and for the fact that I'm not a fucking idiot. Now Aaliyah is asexual and likes videos games and is a bit of a nerd but they started running their mouth about her and I wasn't even fucking AGP style angry I was ass kicking angry. Don't assume I'm a snitch or anything but when he started making dirty references to me I told our teacher, she was pissed because she likes me, and I'm almost as tall as she is and she couldn't believe it was coming from someone like me. IN the end she ends up moving him he shows attitude and she sends him to the office and he goes I'LL BE DELIGHTED TOO. Long story short, a shitload of people hate him mainly including the adults at our school and they take shit seriously and he has threatened to kill someone before so he will be expelled for the columbine style shooting threats. Long story short, I don't have sympathy for the mentally-ill so if I go through one day without slapping him across the face with a book, he won't attend school for the rest of the year. Not that it matters to him but for him to spend extra time with his family that despises him for obvious reasons I can sleep peacefully at night.

/r/AskReddit Thread