Evil is the willful harming of one human being by another.
Notes:
1) Harming is different from hurting; “to harm” is here defined as “to cause lasting damage to the body, life, or livelihood against the will of the one so damaged”. “To hurt”, in contrast, is here defined as “to cause physical or emotional pain”. Thus, BDSM and assisted suicide do not fall under the definition of harm.
2) A “human being” constitutes any language-using creature capable of understanding the concept of harm or hurting. This may include some apes and/or whales, and certainly includes hypothetical extraterrestrials with those capabilities.
3) In situations where all options will cause some harm or damage, the one that causes the least harm or damage does not constitute willful harm. Thus, a doctor amputating a limb to prevent death constitutes no willful harm.