Cause people were evil, since Eve and Adam
Adam & Eve were the first people ever created according to Abrahamic religions. Cassidy is a Christian, which is an Abrahamic religion.
So Cassidy is implying people have been violent since ever, humans were born evil.
God gave Eve specific orders to not eat from the knowledge of good and evil tree. Satan came as a snake and deceived Eve. He told her that if she ate the apple her eyes would be opened and she would be as Christ. Well her eyes were opened, she saw evil and good and went to Adam then asked him to eat the apple and he did. That caused them to get kicked out of the Garden of Eden and I read that there was something placed at the opening so no one could ever enter there again.
So, in this sense, the first humans ever did an evil thing by believing the evil mind of Satan, and not the good mind of God.
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