William is the Man in Black.
It's fucking confirmed. Why? The logo's.
When William arrives at the park, the logo for Westworld is the original one used during the flashback when they were first creating the hosts. The flashback when Ford was explaining Arnold to Bernard.
William is obviously in a much older version of the park. There are other hints such as the bullet, the train being newer, and the narratives being different.
This is the only logo you see with William. With every other character on the show, including the MiB, you see the much newer logo:
The actor who plays William has facial features very, very similar to Ed Harris (if you throw 30 years into the mix). So yeah, not a theory anymore.
So the MiB is not Arnold trying to become immortal and set his "creations" loose.
"These violent delights (the guest's gratification within the park) have violent ends..." is from Romeo & Juliet and is perhaps the biggest piece of foreshadowing we've seen yet. Shit is going to get messy.