The way I see It, we cannot subjectively say that Guardians are right or wrong. We know that we have no malicious intent. We are merely defending ourselves and the thing that ultimately allows us to protect ourselves effectively. Now let's consider the enemies. The Vex are attempting to wipe out all life. The Cabal defeat people, and then destroy solar systems, so it's clearly not imperialism. The Fallen are following the Traveller because they lost their powers. Presumably if that were the case for us, we would act similarly, but regardless, it's self defense for us. The Hive is complicated. They are clearly not creatures of the Light. But why they fight us is weird. I feel like neither of us are to blame or at fault. In any case, we aren't Good for killing, but we aren't bad. It's just a necessity to protect ourselves.