That question essentially boils down to a discussion on the metaphysical definitions of "good" and "evil" and whether there are benefits to being "good" if you do not have a judge to reward such behavior or enforce punishment for "evil" acts. My take has recently been to analyze the situation in terms of behavioral dynamics within a human society. Good is defined by your peers and the society that you live in, as is evil. Because Western societies put a lot of value in being able to keep our money safe in a bank it is generally considered evil to rob one. Likewise we consider human sacrifice or pedophilia to be evil and heinous acts, yet the ancient Aztec and Greek societies, respectively, conceived of those acts as being good. The point of being good, then, becomes one of fitting into your society and furthering its agenda, which seems to mostly involve the propagation of the culture that unites the individuals into a society in the first place.
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