Am I the bad guy for believing my girl best friend instead of my gf

So my gf (20) and I(20) have been together for 4 years.Before I met my gf I had a girl best friend. We've been best friends since we were little kids, like maybe when we were 4 years old. I always trust and would never think she would ever try to destroy my relationship with my partner. She is not the type of person to lie or do anything to ruin my relationship with my partner. I met my gf when I was 16 years old. She was wonderful, she knew how to communicate when she had problems with anything in relationships. And it has always been fixed except for my best friend.She says that my best friend is trying to ruin the relationship and is jealous. I think she is jealous because my BFF and I are close. I tell her to shut up about it and move on because at this point she is just trying to ruin the relationship we have between my best friend and I. So when my best friend told me that my gf went to her house and threatened her saying never to talk to me. I want mad, I have never been this mad. I was ok with her complaining about my BFF. My gf was denyin with everything but this one was enough so I broke up with her. Am I a$$hole for believing my girl best friend over my girlfriend.