As Walter is on the phone with Nina, as she tells him that Peter didn't have any tests scheduled, that he didn't come into Massive Dynamic that day, you can see the exact same thing happen to Walter's face. Just as Chris didn't really need to go upstairs and see her daughter (and I can't even talk about the scene that follows, because that is where I can no longer watch, and even thinking about it is probably going to be enough to make me have a hard time sleeping tonight), Walter didn't really need to see Peter kill that shapeshifter. He already knew. The Peter he thought he knew, the boy he was trying so desperately to protect, for whom he was willing to inject and snort just about anything to try and make himself smarter so that he would be able to save him - that Peter no longer exists. Maybe he never did, except in Walter's mind; maybe the Device has changed him, to suit its own purposes; maybe, in the end, we can never really protect those we love no matter what we try to do.
Playing against this story is the slow rapprochement of Peter and Olivia. She realizes that Fauxlivia didn't just victimize her, but Peter, as well. She extends an olive branch. Peter's face when she apologizes to him, when she acknowledges that he was hurt as well...it's hard to reconcile that smile, the way he swallowed hard, with the man who was already cold-bloodedly killing Shapeshifters, was even then covering his tracks to hunt down another. Olivia will have her own moment of realization soon; bad enough, since she had one last year when she learned that Peter wasn't this world's Peter. She's already been through this before, and now she'll have to go through it again.
I feel like we're on the edge of a precipice.