Question

What behavior could be considered to be an act of betrayal?

Answer 1

There are many kinds (can't name them all off the top of my head) 

1. Telling someone you'll keep it a secret and then tell anyway...
2. Always being there for someone and then not
3. When you cheat on your husband/wife or someone you're in a relationship with
4. Using someone just to get some

Those are all I can think of (at the moment). But you can just pick whichever one you want.