I got it from my favorite internet reviewer named Spoony when he shouted it at E3 2010 regarding the new XCOM game. The game was turned into a first-person shooter instead of a strategy game which basically betrayed the fans who enjoyed the older games.
However, I applied that shout in my daily life because I got rid of people who were just excess baggage, otherwise known as my ex-friends. When one of the people who I considered a close friend (and I mean, I really made an effort to try and befriend her because she was lonely and was being constantly bullied in our first year in college) chose to be with my ex-friends who were shallow and hostile to everyone, that's when I just wanted to shout "Betrayal!"
I mean, this former close friend was someone I trusted a lot. She'd be there for me and I'd be there for her. I did a lot of nice things for her and it's a fact that I don't do nice things for anyone at all. She'd help me with my bitterness with girls and I'd go through a lot just so I could help her with her problems or if she needed anything. It's just stupid that this friend chose to be with my ex-friends who have done nothing but complain about everyone who just gives them a glance that they assume is "bitchy".
So I just use "BETRAYAL" as a silly way of expressing myself when something has betrayed me and a lot has betrayed me. Certain classmates. Friends. Girls I liked. My organization. My university. A LOT.
Missed the bus by 10 seconds?
That sociology girl you like suddenly has a boyfriend?
That tourism girl you really like just blocked you out of nowhere?
That same tourism says she's "uncomfortable" talking to you even though she's just being a total cunt?
That communication arts girl who you like suddenly starts being bitchy to you for no apparent reason even though you're supposed to be friends?
Your organization gave you a job that you didn't apply for nor do you want?
Friend who you trusted didn't even say goodbye to you when you were leaving the country and called you "an insensitive jerk"?
Famewhores getting student council positions in your faculty?
BETRAYAL! BE-TRAY-UHL! BETRAYED ME! These people suuuuuck!
Those may or may not be based on personal experience.
It even reached to a point where I could use "BETRAYAL!" in an academic way when I had to give my part of a presentation regarding the Harold Pinter play "The Dumb Waiter". (Possible spoilers here for you play fans) When I had to explain the ending, I shouted "BETRAYAL! BE-TRAY-UHL! BETRAYED HIM! This guy suuuuucks!"
I even use the term on Twitter when I have to. "UST is cancelling Paskuhan? BETRAYAL!"
I've used it so much that my friends have also started using it in their lives. That's just awesome.
So yes, I've been through stuff that I consider betrayals even though some of you might not consider them as such.