When people act shady, allow them. When people betray you, allow them. But NEVER, EVER, sink to their level. Their choices are a direct reflection of who they are, not who you are.
No matter how angry, hurt, or disappointed you may be - do not allow them to make you bitter. Be better. React with love. Sometimes that means retaliating with acceptance and sheer silence.
Whatever it is, make sure it benefits you as a human being instead of lowering you into a position which you would rather not be in. Just because someone's an asshole, does not mean you have to be one too.