Good VS. Evil

I believe that there is no clear cut 'good' or 'evil' definition. People decide that certain actions are good or bad, such as murder, or donating money. But, one can do a good thing, such as donate money, and still do something 'evil' such as make fun of someone. How can this be? For exactly the reason I said in the first sentence. Nobody knows what good or evil actually is. All we know, as human beings, is what is illegal and socially acceptable. As humans, we will act upon impulses to fulfill desires, or survive. Often times, actions to fulfill this may be illegal or morally unacceptable. But, if the consequence isn't that big, or the person doing is indifferent to the punishment that might follow, then they will do it. For example, I feel that Leah Kleppinger returned that wallet solely because she either didn't need/want the money that badly, or that the consequence for keeping the wallet was too big for her. Most likely, however, it was probably a combination of both factors, in addition, maybe, to a desire for social media attention.