Nice :) 

It's not needed to possess mr. Johnson for the final push, he's perfectly capable to cause of all the shit that will happen by himself. It's a nice idea, but I'd rather have mr. Johnson to be the bad guy because he is, not because of an external factor. Of course he thinks he's doing good and he will do whatever it takes and use whatever weapon he can find to keep humanity safe from monsters, which is what makes him dangerous.

But... talking about trapped ghosts gave me another idea for a mission, which may not end well for your character, so we definitely don't have to do this. 
One of Benjamins life missions is to help ghosts move on to the afterlife. It started when he came across a wandering spirit, who had remained on earth so long that it existed with no goal, just going around in the same circle over and over again and not showing any sign of noticing anything of what happened around it. That's when he decided he would help spirits move on, so they wouldn't end up like that.
And he resents the idea of spirits being trapped by something and therefore unable to move on. If he learns about that he'll want to try to free them.