Like most atheists, somewhere along the line he feels God wronged him, and this is some sort of payback.
If he felt that God had wronged him, he wouldn't be an atheist now would he? Common mistake.
Exactly. If atheists truly did not believe in God, they would simply dismiss such discussions with the same ease as one would a discussion about the Tooth Fairy. Instead, you find the typical athiest to be more of an anti-theist, somebody angry at God, hating on God, hating on people who believe in God.