In the comics, there were several instances that the reverse flash came back in time, one time was to kill Iris Allen ... one time was to search for Barry Allen because Barry was his idol and he wanted to meet him - Barry was already dead at that time though and Wally West was the Flash ... Reverse Flash' usual weapon to kill is to vibrate his hands and penetrate the skull of his victim, and this is what he did to Iris ... i would assume that this tv series is one of those times that the reverse flash time travelled