To quote D&D (specifically, 3rd and 3.5 edition)
Good Vs. Evil
Good characters and creatures protect innocent life. Evil characters and creatures debase or destroy innocent life, whether for fun or profit.
"Good" implies altruism, respect for life, and a concern for the dignity of sentient beings. Good characters make personal sacrifices to help others.
"Evil" implies hurting, oppressing, and killing others. Some evil creatures simply have no compassion for others and kill without qualms if doing so is convenient. Others actively pursue evil, killing for sport or out of duty to some evil deity or master.
People who are neutral with respect to good and evil have compunctions against killing the innocent but lack the commitment to make sacrifices to protect or help others. Neutral people are committed to others by personal relationships.
Being good or evil can be a conscious choice. For most people, though, being good or evil is an attitude that one recognizes but does not choose. Being neutral on the good-evil axis usually represents a lack of commitment one way or the other, but for some it represents a positive commitment to a balanced view. While acknowledging that good and evil are objective states, not just opinions, these folk maintain that a balance between the two is the proper place for people, or at least for them.
Animals and other creatures incapable of moral action are neutral rather than good or evil. Even deadly vipers and tigers that eat people are neutral because they lack the capacity for morally right or wrong behavior.
I really, really like how the writers of D&D put Good and Evil (Law and Chaos, not so much). Basically, good means protecting the innocent, even at the cost of yourself. Evil means hurting, oppressing, and killing others.
This means it's about intent
and actions. Of course, it also means nearly everything is in the grey area. You want to feed the homeless? Great! That's good! Except that you're feeding them the flesh of the innocent... (beef)
One of the best stories I've ever read about good and evil is
Looking for Group. It's a work in progress, but one of the very first comics demonstrates this view of Good and Evil perfectly.
Richard, evil undead sorcerer, kill someone, horribly, in front of Cale, a sickeningly sweet good guy. Cale tries to shoot Richard, only Richard grabs some dude and shields himself with their bodies. Cale was trying to do good (punish an obvious evil-doer), but ends up doing evil (and is heartbroken over it). Through the course of the story, Cale gets more and more used to the casual atrocities committed in front of him (but still generally tries to be a good guy), but he also gets more and more involved in the Greater Good, trying to set up a system where those atrocities
don't happen, even if that means he has to commit a few himself along the way.
To quote Ash, from Evil Dead:
"Good, Bad... I'm the guy with the Gun"History has shown us that might really does make right. Joan of Arc was famous for, amongst other things, being pretty good at killing people. Quite a few "good guys" in history is a good guy because they're really, really good at killing people (or worse, really good at convincing *others* to kill!). And yet, killing is bad? Killing is wrong? Then why do we respect people like Churchhill or Patton? I'll give you a hint: It's because they won.
If you kill 100 people, you're a psychopath. If you kill 100,000 people, you're a hero. Funny how that works, eh?