Come to the dark side, Luke - Taylor Stewart

Nothing is as simple as good and evil. Two sides - complete opposites - with one having completely altruistic motivations, and the other wholly inundated with wicked intentions? That doesn't happen. It's never happened. It won't happen. That's why argument and rhetoric and persuasion exists, why we learn what we do in English class. No ideas are completely impenetrable, and all are subject to scrutiny and opposition. With the Star Wars example, as well as most other like films, this holds true. There's a reason many viewers feel pity for Anakin, why we imagine what could've been.
As for absolute good and evil, it all lies in the eye of the beholder. Examples throughout history - Hitler, Stalin, Trotsky, Amin - they all believed the atrocities they were committing were being done for the greater good - for their good purpose. Whether it be for religious purification of political domination, their actions were purely upstanding and ethical in their own views. A consensus on something's status as good or evil cannot possibly be reached communally; but we can certainly form our own opinions on it, and depending on all the factors that play into an individual's life, these opinions have the ability to be radically different.
Having so much gray area in things - people, conflicts, ideas - can make things seem abstruse and difficult to understand. But sometimes that's what makes it interesting.