This is what I meant.
Apparently Man in Black was there to witness Arnold dying, or stopped him from blowing up the whole place (have to hear the line again).
So if this is William's first time in the park, and Arnold has already killed himself, how can the Man in Black be there to intervene like he did?
We don't know that MiB ever met Arnold, only that he knows who he is now. He knows he died in Westworld and that he planned it. There's no indication that he witnessed it. He just wants to know what Arnold left behind. When Ford and Dolores speak, she says that the last thing Arnold said to her before he died was that she was going to help him to destroy Westworld. I think that's what the maze is. What it has always been. The endgame is apocalypse. William will prevent the first attempt. MiB is back to help trigger the second attempt.