I see it as negative thoughts and action brings more negativity, doesn't necessarily means bad things happen to you but rather that your life will be more negative as a result. Someone mentioned Hitler, Mao, etc. I seriously doubt those guys were living a rich and full life and enjoying everything they did. Most of them were paranoid up to the day they died and thats not a good life.
Positive thought and action breeds a more positive life. That doesn't mean something bad won't happen, but it does mean that you can find the silver lining of a bad event easier than you could with negative thoughts/actions. Its a perspective thing. Its not necessarily a force that judges you on whether or not you shoplifted that twinkie last week (I SAW YOU!!!), but rather the nagging thought later that pervades your mind. Guilt is the biggest form of negative "karma". It will kill you in the end, and make your life miserable unless you do something to change it and that involves a positive action, even if the consequences aren't necessarily pleasant at the time.
What you look for is what you find, what you do will reflect back on you and affect your life.
And hey if you can believe that your actions don't matter and you can do whatever the fuck you want, go ahead.