Man, fabulous episode this week--I'm definitely digging the season more and more as it gets deeper (and I never disliked it to begin with, sooo).
My theory: we're going to see a genuine multiverse, and this Jay is one of a lot--perhaps, say, 52? He's been traveling Earth to Earth, murdering his counterparts one by one to steal their speed (when the resident Jay is the Flash of a given universe). Mask-dude is probably a speedless Jay from some universe.
Alternate theory (that I think some people here kinda touched on): Jay has been diffused or separated somehow; not all parts of him are necessarily aware of this. Multiple personality syndrome on a physical scale. One of them genuinely dying could have. . . effects.