Why do friends side with the person who did the abusing in the relationship?

Like my ex girlfriend constantly made fun of my condition, when I would get anxious about certain things she made fun of that. She made fun of my family. She was physically abusive towards me smacking me twice in the head, she was abusive towards our dog to the point where he has night terrors she also has hit him and slammed him on his crate leaving a dent in it. But all her friends see no issue with this and seen me as the problem. I never called her names I never physically struck her I was raised better. Even when she made me upset I was still there for her in need.

Why do friends side with the person who did the abusing in the relationship?
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