If you kill a cow to eat is good for you, but if the cow was milking a younger one, you are bad because you killed his/her mother.
Good/evil changes in accordance to the perspective of each one.

I agree with the 1st statement. I disagree with the 2nd statement (unless applied to a sociopath or similar). If the 2nd statement was true for normal people, some people would feel it is ok to kill a cow while she is still suckling her calves.

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Extraordinary claims require extraordinary evidence - Carl Sagan