Light and darkness are defined by each other. There is no bad or good about either one for they are both necessary for the existence of the other. To illustrate this, define what a shadow is. It's the absence of light and light is the absence of darkness. The only thing that we as guardians are fighting for is survival, which is pretty much the same thing the Fallen, the Vex and the Hive are all fighting for. None of these can really be called Evil in any objective sense of the word.