people act according to their own instincts, however as crazy said before it's due to a universally set moral code why some things are viewed as as evil..

but universal moral codes dont really exist as it depends heavily on the culture of the society

like some asian cultures see it as reasonable to kill family member who bring shame to their family, whereas other cultures would see that as evil

the Nazi society saw fit to attempt to commit genocide against Jews, which according to their moral codes was fine but the rest of the world saw as evil

it depends on the perspective