One of the most popular theories out there right now concerning the question ''Who Is Zoom?'' is that Henry Allen is indeed the man behind the monstrous mask. Here I'm going to explain how I think Henry Allen came to be Zoom in the first place.
Back in the finale of season 1 we saw Barry travel back in time to save his mother. As he was about to save her future Barry stopped him at the last second. Maybe over at earth 2 future Barry does indeed accomplish his mission and save his mother therefore resetting the timeline for earth 2. Some of the changes that were made due to his mother still being alive could be that Reverse Flash, instead of killing Nora, actually kills Barry.
We know that earth 2 Harrison Wells is responsible for creating Zoom, but we don't know why he created him or how. Was it the particle accelerator? That seems the most likely way, but there could be more to it than just an experiment gone wrong. Maybe, like Barry and evil Harrison Wells in earth 1, earth 2 Wells trains Henry, but then somewhere down the line something goes horribly wrong between the two and this turns Henry over the dark side.
As Barry was seeing his past/present/future when he was going back in time during the season finale we saw The Flash museum. Maybe Henry somehow learns that there are other speedsters just like him in his world and other worlds. He then sees that they are more successful/more appreciated/more popular than he has ever been therefore causing him to envy the other Flashes. Envy and Jealousy could be Henry's motivation for wanting to kill all Flashes; he wants to be the only speedster in any world to feel special and to feel appreciated.
Finally, as you can see in this picture Barry and Zoom's physique are very different. Zoom has more meat on him and is more muscular whereas Barry is skinny compared to Zoom. The only way I see earth 2 Barry as Zoom if it's a future version of Earth 2 Barry where he gained more muscle.