It's a great question. Adherents to the Old Testament point to Original Sin.

But even without religion we can use today's standards of morality to be sure that at some point in human development there was a first time for a person to commit an abjectly "sinful" crime of inhumanity against another. Not having been there to witness the event, I can't tell you the what or why of it. Fratricide, parricide, abuse, and more, I think we can be sure there is a first time for all of these.

Speculation leads to many answers. One, absent religion, is that because we seem to be derived from pond scum "inhumanity" developed somewhat before morals and ethics did, and has found its niche in us.