I believe the story is open ended, people seems to hate it online but honestly I prefer it. You can interpret the ending however you want and personally I think satoko died during that time. I believe he's hallucinating and the guard was instructed to act this way by satoko before she died, or maybe he acted this way on his own will. The whole letter thing could be his imagination as well since it's his first time coming back after all those years, he's thinking of what ifs before dying
