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

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.