Relativity of ethics
What is good or bad? Something that is bad for you might be good for someone else... Imagine for a moment, a parent stealing bread from a shop to feed their starving child. This is obviously bad for the baker, however it is good for the parent and child. It seems in our culture, we use the words good and bad as common terms, but their meaning is actually relative to the individual at that point in time. This is the concept of Relativity of Ethics (or moral relativism).
Another example is Star Wars. In Episode IV, the hero, Luke Skywalker and the Rebels go out to defeat the Empire and blow up the 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? Or freedom fighters?
The Definition of Good
In this galaxy, not so far away... under our 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 environment as this is the foundation of our collective need, see Maslow Hierarchy...