Am I the asshole for wanting to cut ties with my best friend who is in a toxic relationship?

My best friend is in a toxic relationship with a guy I really dislike. Interestingly, he actually hit on me first, but I shut him down. Shortly after, he went after my friend, and she took the bait. They’ve only been dating for a month and a half, but they’ve already had constant fights and a lot of drama. He doesn’t show her off, he deletes her comments on his posts, and he hides his social media activity from her to talk to other girls.
Today, a video of him popped up on my feed where he says: "The best part of a relationship is when you start manipulating them, they slowly distance themselves from their friends, and now they believe that you are their only safe place."
It’s terrifyingly accurate. I’ve drifted away from her ever since they started dating. From the beginning, she told me that she would always prioritize her boyfriends over her family and friends, and I’m finally realizing how true that is. I’ve distanced myself so much that I don’t even want to go to her birthday party anymore. She’s planning a sleepover and really wants me there, but honestly, I don't want to go. It honestly bothers me so much to see how she acts when she's with him.
Am I in the wrong here? Am I just being selfish? I sometimes feel guilty because I’m a very individualistic person, and the moment I’m not comfortable with a situation, I tend to pull away and leave people behind because I don’t like carrying other people's baggage.