If he goes on a rampage over the next two episodes, it'll feel out of nowhere, I fear.
I don't think it will be at all. We've heard the entire time that he's capable of more than what we've been shown (not just from him, but essentially everyone we've met up with who knew Joel before the events of the show), but unlike the game where that plot point is at odds with the gameplay, I think it'll make a hell of a lot more sense to see him snap in the show. He clearly doesn't
want to be that person anymore and when we meet up with him and Tess, he doesn't
need to be. They live in the QZ, and as shitty as it might be they're long past needing to kill to survive. Unlike in the game, his age clearly effects him more in the show; he's no Rambo.
We saw a glimpse of ye olde Joel in the first episode when he beat the guard to death, and the next episode should push him further than that moment ever did.
That being said, I've always been a fan of less is more, so I don't need to see Joel massacring people every episode. Show me that once, have it be good, and it'll be
far more effective than showing that to me throughout 8 or 9 episodes and having that impact wear off by the time it
really matters.