Betrayal

I don't know why these thoughts and memories are sticking with me so much right now.
I've had friends betray me, and, well, I thought I was over it.
I've heard people say there's no such thing as closure... but I just want to understand what would make people pretend to be your friends, all the while talking badly behind your back, putting you down, and never even telling you what they really think? What would make them suddenly decide to gang up on you- online, no less, the most cowardly form of attack out there- and attempt to tear you into pieces?
I thought I could trust these people, even though I didn't always care for their attitudes about some things.
Some of the pain, though, is because I lost one of my closest Baha'i friends in the process. Even though she and I are now friendly towards one another, and she swears she really wasn't a part of it... I just can't bring myself to trust her anymore.