bad and evil , like good or nice are nothing more then words that decribe actions or thoughts. what causes anything a person does is many things. it can be anything from a persons background, their genitics, mental health, etc. i never met an athiest that would say good and evil don't exist but an athiest might say good and bad instead of evil. what causes you to wonder so much about how athiests think if you believe in god like you seem to want us to think? are you having doubts perhaps?