Other People's Friends Are Not Always My Friends

Just made a new friend recently about 2 1/2 weeks ago, through a good mutual friend. Yeah, it was on Facebook. Funny thing about other people's friends. They should remain just that.
Sometimes a small circle gets along very well, until an incident occurs amongst them. I was just a virtual online bystander, wasn't involved, except for the flood of messages from the 2 who were there, one of whom was the receiver of the incident in question, & who both asked me for some advice, which I gave them this past weekend.
I am a new friend. But the incident in question, which will not be discussed here, caused me to delete the brand new friend, yesterday. Then I thought better of it this morning, after I'd learned she'd apologized, and re-added her. Had it not been for the snotty announcement on her "what's on your mind" thought for the day, I would have just commented that I was glad things were getting sorted. Especially since she re-added me. After reading her b*tchy & self-serving commentary, my good thoughts changed, and added my own superior thoughts. Yeah, she deserved what I said. I deleted her for good.
I then told the mutual friend what I'd done, and said- "she can bloody F***K OFF AND DIE," adding, "I'm sorry." I really am.
I didn't know her as well as our mutual friend, so I'm not that upset. But I know our mutual friend probably is disappointed that I don't approve of her, his "best m8", as he thinks of her, not only for the incident that occurred, but for the sheer childishness of her statements after I re-added her.
Yes, I shouldn't have deleted her after the myriad messaging from the mutual friends yesterday. I should have left it alone. But I was so shocked at the opposite of what our mutual friend had said of her, that I still can't reconcile the two extremes. Particularly when I know what happened. The day after the incident, she was happy and crowing about what she did on her home page. It was a true criminal act, that is usually paid for with jail time.
I've learned my lesson. I will stick to the friends whom I select, that I know from the test of time are my true friends.