Greg Stillson wasn't kidding about the Armageddon destiny thing.
He saw what Johnny saw when they shook hands and at first dismissed it as a hallucination. But then during one night he read about Johnny's exploits as being able to see the past and future. Rather than try to avert this, he believes Because Destiny Says So
, so he tries to make it happen. Of course Johnny believing otherwise
does what he can to stop it.