I'm not a Athiest I believe in God,
but...take my friend Izzy as an example. She doesn't believe in God because she just never has. She doesn't believe in anything at all spiritual or religious. She doesn't know why...she just doesn't.
Just because people feel bad about things doesn't mean it has anything to do with God.
Say if someone hit someone they would feel bad about it because they gave the other person a bad feeling of pain and they don't like that they made them hurt.
There doesn't have to be a God. Like some people worship the Sun, some people worship goddesses.
When you said
Therefore, there MUST be a God
You should of said "Therefore there must be something out there." You don't know, nor does anyone, that God does things, some people choose to believe he does and some people believe it's something else. Some people believe its just scientific and its out feelings and actions that make good and bad not God and the Devil.
Who knows? Answer = No one.
It doesn't HAVE to be God, it COULD be something else.