"Someone who claims to be a Calvinist was arguing that God is the source of evil."
If not God, then who or what? Are you saying that evil is uncreated?
What I reported was what John Calvin wrote and taught--God is not the creator of evil. If this point of Calvin's raises more questions for you, and you consider yourself a Calvinist, I suggest that you read a lot more of Calvin's writings. You may find the answer you are looking for there.
Evil in the biblical sense can only be understood as something immoral. God is not a source of immorality. Evil "exists" only because of our immoral choices, whenever we reject God.