So how is a said, non-existent being, God, supposed to be the greatest source of evil?

[LilChicken22]

Evil is a subjective term and relative since its definition differs through the history of mankind. One considers some things to be evil while others do not. Actually evil/good doesn't exists. It also differs per species what evil is. Some species of primates, 50% of pregnancy occurs by rape. Although we consider rape as evil, for that species it is necessary to survive since evolution 'designed' it that way.