The back and forth nature of this type of argument proves my point most of all - that man is basically evil.
I don't agree with. Man lacking a moral philosophy can be evil, but one does not need religion to adopt a moral philosophy.
True, but does an ethical system make sense on its own grounds unaided by divine sanction or support when Glaucon and Adeimantus' problem is set before it, or when the agent is in the position of a wearer of the ring of Gyges?
My contention was that man can be evil with or without religion itself, so it is pointless for those on both sides of the debate to continue trying to one up the other side with examples of evil committed by "religious" or "non-religious" people that were essentially charasmatic leaders with a lot of power.
Your moral philosophy is the key and is in reality what I was referring to. I think it can exist with or without religion. Just as evil exists with or without religion, so does good - your moral philosophy so to speak. It is the moral philosophy that you speak of that is ultimately responsible for man's actions towards others and their evil or good label that I am applying.
In some cases, religion provides a basic foundation or understanding for people (as in "What would Christ or Buddha do?"). In others, it is used to commit or entice others to commit evil ("Allah or God says kill the Infidels / non-believers"). But you also have the "Religion is Competition with the Supreme State / My Power so it needs to be destroyed or suppressed" folks as well.
I would argue that without religion, man would find something else to bicker and kill each other over because many do not possess the moral philosophy of which you speak.