There are several theories. One is simple pragmatism, in which one accents the practical outcomes of things. In this model, if an action works, it would be good. A related but more complex theory would be utilitarianism; if more people would benefit by you bribing someone than not, the action would be good. Moral relativism would argue that right and wrong depend on where you are standing. What's right for one might be wrong for another, including a bribe. A position of moral absolutism, by contrast, would argue that such an action is always wrong.