I have this group of online friends which are around my age (17~18) who I met around November of last year. We all live in the same country. Recently, two of the members of the group, R and T, started dating each other. R is a very shy and insecure boy. He's never been in a relationship before. T is a troubled girl who has had family problems in the past which have 'messed with' her emotional side. She has been in multiple relationships (6 IIRC) in the past 2 years, which have all been LDRs. The last relationship she was in, was with an American boy and lasted roughly a year (let's call him K).  
  
Right before December ended I got invited to spend New Years with them. I accepted this invitation. Nothing strange happened during New Year's, other than the fact that out of nowhere when we were going for a walk, R and T started talking to each other (they weren't close before, so it was kinda strange). When we got to one of their homes to just chill out, there were a solid 5 minutes where I just remember they were laying down on top of each other cuddling. Anyway, fast forward to after New Years, they all of a sudden get really close. Close to the point where they started hanging out in private Discord calls instead of joining the voice channels on our server. They started seeing each other quite often in real life, as well as he would pick her up from school and walk her home every day. This was really strange considering he NEVER left his house before. They started napping together, and she even left a hickey on his neck.  
  
So here's my problem with this. While all of this was happening, she was still dating K. Even worse K had no clue that T and R were doing things with each other. T and K still considered each other significant others during this time. In my eyes, I see what she did as cheating and I'm not okay with it. I did not like the way that either R or T handled the situation, because, at the end of the day, K is a human being and no one deserves to be cheated on. T broke up with K, but only after she had been doing stuff with R for about 2 months.  
  
When I confronted her about this, right after their breakup, she told me that her and K "weren't really together and we're just together for the sake of not being alone". Tbh this sounds like a really lousy excuse, and also, K was really devastated. Taking into consideration all of these details, I don't know what happened inside of me, but I just couldn't take it anymore. I lashed out against them and started acting really cold to both. I don't agree with their relationship especially how it started. The whole "pushing all of their friends to the side" and "cheating on her boyfriend" was just too much. I would understand if it were only me thinking this way, but most of the people in the friend's group were disappointed in both of them as well. So I would like to know, am I an asshole for feeling this way/being cold to them because of it?