When Greg Hart receives a letter telling him not to go to Myrth, he is happy to oblige, especially since he is destined to die there if he returns. But as Kristin Wenslow points out, prophecies cannot be changed. Simon didn't say Greg WOULD be killed if he went back. He said he WILL be killed. Greg has already survived two trips to Myrth, but how can he be expected to survive a third? He can't. The unalterable prophecy has already foretold his doom.