How much time was between the 1st and 2nd shot? I'll reserve judgment until that tidbit is known. And, gunshots are loud enough that there should be earwitnesses.
Story is very poorly written, so I won't base judgment upon those "facts." Almost seems as if the story was written in a way to intentionally make it appear as bad as possible. If it's true, in that timeline - she had left, was downed in an alley, and begged for her life between the 1st and 2nd shot (as per the OPs commentary and the order of facts in the article), then very clearly murder. As much as I sympathize with him, I'd send him straight to prison.
edit: after reading other articles, it seems she said she was pregnant before either shot.
edit edit: she stumbled in the alley before pleading for her life, then was shot twice in the back. Guy had been robbed before, guy had just been severely assaulted (maybe for a 26 year old guy, being tackled isn't severely, but for an 80 year old man, absolutely); so I'd be happy if he was charged with manslaughter, not premeditated murder.