Apologies; this is going to be long. Also, this is a throwaway because most of the people on this quote are on Reddit. I will try to be as objective as possible and not exonerate or vilify either me or the other guy. 

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Both me and the guy are in first year university right now. Most of the stuff in this post occurred when we were in grade 12 (in the same school). Letters are hard to keep track of, so let's call him "That Guy" (DnD players, you will see why if you read on). 

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I first met That Guy when we were both in grade eleven, and although he never fully integrated into my friend group, I was pretty friendly with him and we often talked about random stuff like history (we were both interested) and games. I noticed that he was not the best at some aspects of communication; he would do things like interrupt people (I also had that habit but I fixed it after realizing), lying about trivial things, needlessly bragging, pretending not to hear people if he didn't want to hear what they had to say, and being slightly disrespectful to teachers.

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Some examples: 

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He always lied about his grades; one time I literally saw that he got around 50% for a math test (by accident, since he left it on a table) while he bragged about a 96% to everyone. I later heard him complaining to the teacher about it. He boasted that he had many weapons at home (this might not be a lie, but I doubt he does because some of his friends went over to his house and didn't see any) and that his house's fence was "impenetrable" after sending me a picture of a fence that was from Google Images. He also pretended to know about things that he didn't, and one time, when I was clearly correct in an argument, he put on his headphones and pretended not to hear me. I tested him by asking what music he was listening to, and lo and behold, he could hear me and answered. He also didn't pay attention in class and refused to apologize to teachers when they called him out for it, even when they got legitimately angry. One time our English teacher (whom I liked, since English was my favourite subject) got pretty upset with him and his response was to discuss how English should be removed from the curriculum at lunch.

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I'm not saying I'm perfect either. Often, I continued to provoke him when I knew it would escalate a debate further, and sometimes I purposely asked him questions that I knew about but he didn't just to see his reaction. Still, we were on relatively good terms with each other. 

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The problems began after two separate incidents. One was when he ruined a long-running DnD campaign that included me and one of my close friends by adding overpowered characters that completely derailed the plot and attempting to control other people's characters (he tried to kill most of the other PCs then got angry when his was put into fatal situations). Both my friend and I were quite angry about that. Remember that friend; let's call him Cool Guy. 

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The second was quite ridiculous when I look back on it. I helped him for a physics project even though I was not part of his group, and he began to ask me for help on his other assignments as well. I was fine with this until it got to his personal profile for university application. It was horribly written, was based on the standards from the wrong year, had blatant lies about his achievements, and included him claiming to own the DeviantArt account of a talented artist who was most definitely not him, since the artist had a twitter account that contained his photos and the fact that he lived in America (we live in Canada). 

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I found this hilarious and openly mocked him about it. That was probably a douche move. He reacted by telling me not to tell anyone, which I promised to, but when I jokingly mentioned his personal profile offhandedly in a Discord server, he forced the owner of the server, whom we knew irl, to ban me from it and not add me back. I was quite pissed off about this because I never planned on showing anyone, but now I did. I showed everyone in my friend group, and they secretly showed other people they knew. That Guy was clueless about the fact that his personal profile got distributed like so.

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Afterwards, I got interested in the life of a Nazi official, since he was the only high-ranking Nazi to be successfully assassinated and I love assassination stories (especially when they're justified). That Guy got very curious about this and tried to figure out who the man was - I would have told him, but he somehow got the conclusion that I was in love with the man despite the fact that he was evil, dead, and married. I am also asexual and almost aromantic. I tried to explain this to him multiple times, but he believed that having interest in someone's life meant that you had to have a crush on them.

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Finally, he offered to tell me who he had a crush on if I told him who the man's name was. I agreed; since he was also telling me something, I could get him to promise not to tell other people. Less than a week later, the guy that he had a crush on (whom I was also acquainted with) asked me if I was in love with a Nazi. After I told him that this was not true, I asked him where he got the information from, and he told me that it was That Guy. I then told the dude that That Guy had a crush on him, and he ended up telling a lot of people as well. That Guy was clueless again.

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I then privately confronted That Guy and told him that I would leak his personal profile and also the identity of his crush if   he ever talked about this again. As far as I knew, he didn't. 

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I have been avoiding him after that, but he keeps approaching my friend group whenever he sees us around campus. One time, I was drawing random stuff with Cool Guy when he appeared, and he told Cool Guy to draw a face based on his description. After Cool Guy did that, That Guy turned the face into an anti-semitic caricature and laughed at it for a solid minute before I left the room. By the way, this was in my sketchbook and in pen, so I had to scribble it out.

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He also keeps asking Cool Guy for his homework so that he can copy it, and Cool Guy was so nice that he agreed until I persuaded him to stop helping him at all. In context, it's a CPSC assignment and he refused two people's (one of which was Cool Guy, the other was another close friend) help to actually explain it to him so that he could do it.

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This is annoying and all, and I have been very curt to him whenever I see him, but perhaps I am in the wrong since I blackmailed and lied to him. However, he also slandered me and broke his promise. Who is the asshole here?

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Thanks for reading this long-ass post.

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TL;DR I leaked sensitive info about this guy after he got me banned from a Discord group, then leaked a secret he told me after he told everyone my secret and also slandered me. I blackmailed him with information that I already told everyone. He is not the best person, but neither am I.