So whose DNA host brain did Bernard snatch?
I think we can effectively rule out Ford who we found out this episode has implanted himself into the cradle which is the brain of the entire park...
Only two theories make sense...
That human-host hybrid brain Bernard snatched from the lab is Arnolds and that's why he wakes up on that beach all confused...It also explains why Delores is testing him for fidelity like William was doing Delos...We had two scenes with her testing Bernard/Arnold first episode of the season and this last one...She loves Arnold and no one knows he's dead other than her...Makes sense she would try to resurrect him and use him as her ticket out of the park...
MIB/William is the other theory which there are a plethora of clues that imply he's not WHAT he appears to be...Hes been chasing a game Ford specifically told him is only meant for the hosts...
Ford to MIB via Lawrence daughter said:
You still don't understand the real game we are playing here," Ford said (via Lawrence's host-daughter). "If you're looking forward, you are looking in the wrong direction."
By telling him to look backwards Ford has to be referring to an event in the past like say Williams death...
Young Ford to MIB said:
"The game begins where you end," Ford said. "It ends where you began."
The game began when William died...It ends when he successfully traces back his steps to discover what he now has become, a human-host hybrid...
Like I said in another post, telling Delos what he is caused the host body to reject the human DNA brain every time...It makes sense Ford figured out another way, let the subject discover what he is on his own, the same game designed for the hosts...Trauma is probably a key to this also as it is for the hosts...Hence why MIB daughter has been thrown in the mix...It's not a coincidence she found him unscathed and MIB knows that...