I slept with my boyfriends ex

So I moved out of state with my ex we were good until things weren’t. Our relationship was toxic with being emotionally and verbally abusive. Even his family saw it but only on his side and thought I was the evil person which is false. My ex liked to drink ALOT which I didn’t mind because I would join in at times until it wasn’t fun any more. I flew back to my home town to be with family. He cheated on me and then dumped me. With all my belongings still with him in the state that I moved to. He’s shipping everything back to me but I ended up sleeping with his best friend a few days after him dumping me. I mean this guy was like my ex’s brother. However, I did nothing out of spite. Should I feel bad about this?