Good an evil is a measurement, much like math. It's applied to whatever it's measured, the same as me saying "I have two apples, I add another apple and I have 3". I use math, the same way that people apply good and bad to other people. However, good and evil is just a concept, break a human down to a molecular level and it's not broken into good and evil, because good and evil only exists as a concept. Take away these arbitrary notions (which change from culture to culture and person to person) and we can start looking at why people behave the way they do, instead of asserting arbitrary measurements of our approval of their actions.
tl:dr - good and evil doesn't exist.