I disagree. That's a bit cavalier to suggest that he has no humanity. If that were the case, he probably wouldn't have thought twice about both intimidating and shooting the clerk, let alone throwing Peter the chocolate milk. Moral ambiguity is a common theme in literature and film, and it's commonplace these days anyway. We hardly ever see villains doing things 'in the name of evil' anymore, it's just not very belevable. In many cases, even when characters are outright scoundrels, they're usually portrayed as insane or psychotic to varying degrees. Heck, even in comics ambiguity is the common trend. Look at how the Brotherhood of Evil Mutants dropped the 'Evil' from their name as time went on.
As for the scene in question, it may or may not have been an accident; we can't come to anything conclusive from what was shown. Now, a 1911 won't just go off, but the robber could have squeezed the trigger as a reaction to the calamity. Who knows? I think the entire scene just didn't give us enough. You can't say with any certainty that the guy was a violent thug, as his reluctance to pull the gun on the cashier suggests otherwise. The robbery itself told me that the guy was just a trouble maker, not even trying to so much as intimidate the store clerk. Then again, he had a concealed weapon on him and was fleeing the scene in the first place, so who's to say?
Agreed, but I think it would be more difficult to pull off on film, especially if Stone becomes a fan favorite. It could be done, but it would take some clever writing.