Relativity of ethics
Relativity of ethics (or moral relativism) is the concept that something that is deemed good might not be good for everyone. i.e. the words good or bad are relative terms and their meaning is entirely dependent on their contextual framing.
A great example of this is Star Wars. In Episode IV, the hero, Luke Skywalker and the Rebels go out to defeat the Empire and blow up their nefarious Death Star. However flip the perspective, imagine an Imperial officer coming to work on the Death Star simply to feed his newborn baby... It begs the question, is Luke doing good when he launched the proton torpedo blowing up the Death Star or was the Imperial officer doing bad when coming to work to feed his newborn child? Isn't the Star Wars Saga just a story about a group of heartless terrorists?
The Definition of Good
In this galaxy, not so far away... under your feet in fact, we all stand on earth. We all drink the same water, breathe the same air, if someone truly thought their aim was to do good, the only 100% good, would be what is in the common good for all. i.e. their actions would primarily be involved in protecting our common ecology as this is the common to us all. Once this relativity is understood, one can start to see who the real bad and good actors are.