chess is everything for him he gave it his life. I wonder how you will react when you deeply convinced that what is the dearest for you has been taken away by what you believe is 100% a cheater?
If you are deeply convinced someone is a cheater why not come out with a proper, official statement? Resigning like this is immature and ridiculous.
its hard to prove. lets say you played millions of games, you deeply understand how a human mind plays chess but you can't write it down you just know it because it is your art and you practice it everyday for the major part of your life.
How do you explain to someone else this conviction?
This has been answered many times. He probably has deals regarding what kind of public statements he can make for a while, in connection with the sale of his company.