THAT WAS UNEXPECTED. Thoughts/spoilers(?) below.
Maybe he ran away because Robbie/Cora told him about their realization about Hero (I’m guessing he disappeared shortly after they were released.). I mean, finding your actions could have indirectly killed someone is a lot to deal with, especially when you’re a teenager. I don’t think he felt guilty at all about what he did before, but I also don’t think he ever thought his plan would cause something so extreme to happen.