My theory is still along the lines of a heavily damaged mitch unit that has slowly gone insane over its many years of isolation from the rest of the sentinels. There was an episode of star trek voyager that comes to mind, they found a hologram as the lone survivor on a ship that everyone sympathized with and seemed harmless, turned out it was insane and was trying to kill everyone. Im thinking something similar will happen here and greenie will join with it.