Maybe these people aren't mentally sane. Maybe they didn't die. You can't really know without being the person. The same could be said for the people claiming to see heaven. It's not proof of anything. For all I know it could have been a crazy, and bad dream. Regardless, this is not even close...
It seems to me that there cannot be any correct religions, since most people just choose what feels right, and/or follow the religion of their families. There is no right path, in my opinion, except to be the human that you were made to be. Animals don't seem to follow religions. Why should you...
Perhaps this problematic world is the way things are supposed to be. We may not perceive things like "sadness" as good, but those are just our personal feelings on the subject. The benefit could be a world that balances itself naturally. Other than that, I don't really know why we are here.