Depravity goes beyond mere bad behavior — it is a total lack of morals, values, and even regard for other living things, like the depravity of a serial killer.
You can see the verb deprave in depravity. To deprave is to make something bad, often to the point of moral corruption, like the parental fear that a bad influence will deprave their good kids. So, depravity is behavior that is morally corrupt or otherwise deemed wicked. Don't confuse depravity with deprivation, meaning "being denied something that is necessary."