I had sex with my old besties ex

So I used to be very close to this girl, during January she and her boyfriend broke up after being on and off for three years, I secretly sided with him because she always treated him poorly. Anyways, I’m very close with him as friends. At the beginning of February I broke up with my boyfriend because he started calling me a whore and accusing me of sending nudes to other guys when I never did. So my “friend” sided with him when we broke up and never really talked to me, she changed a lot over time. My ex and I are now friends and we get along fine, however my friend and I haven’t talked and I don’t even want to talk to her. So last weekend her ex and a couple other people came to my house and I had sex with her ex. So she found out and she wants to kill me. But everyone in the same friend group thinks she had absolutely no reason to be angry with me