What determines whether actions are good or bad?

In a large number of religions, 'good' and 'bad' are defined by some all-powerful diety who either sends a representative or himself to tell us about what exactly they want us mortals to do. In Buddhism, instead an enlightened individual basically worked it all out. So the basic question is - why does that happen? What decided that killing (even if your intentions are good*1) will cause something negative ? What about the other actions? There is no 'final judge' to confirm or deny we've followed a particular path - so why does the universe (?) act this way? And what makes us sure that we're correct? *1 - It's an impression I got from reading some other answers, if I'm wrong on this please do correct me)