The fundamental principle is that G-d had a purpose for Adam. Adam failed, which was disastrous both for himself and for the entire world. G-d decided that such power will no longer belong to a single individual, so He took Adam's soul and fragmented it into many different souls. All men come from Adam and all women come from Chava. The evil inclination was originally created with enormous power, but was very small. After Adam's sin, the evil inclination became very great when it became interwoven through all of creation, which is called tigbores hara. When the evil becomes greater, it is now necessary to remove the corruption through action.