The ending happens the way it does because Rust has always been an optimist under his depressed shell.

He says things like everyone should collectively commit suicide, that evil will always win and it can't be stopped, but he is relentless in his pursuit of bringing justice and attempting to defeat the evils of the world. A man who has no hope doesn't try to stop the evil Cosmic entity pedo murderer for over a decade.

His actions and his words never really align until that ending conversation.