Why do we blame Satan for Evil when Satan himself was created by God?

Satan was not a part of H'braic theology until the Babylonian exile. They invented him patterned on the dualism they encountered while in captivity, in order to have someone besides YHVH to blame evil on. Sure, it raises all these questions, but the alternative was to continue to blame YHVH for evil, which they did up to that point, like when they said YHVH incited Hch'v'd to enumerate Isr'el. They liked the idea of an evil demiurge better, and Satan was born.