It seems silly that something as seemingly meaningless as Batman displaying a new haircut can end up being so jarring, but...it was! Bruce Wayne with a John Cena buzzcut? Weird. It's one of the excellent ways Greg Capullo captures the difference between the young Batman of Zero Year and the more seasoned crime fighter we'd been seeing in The Court Of Owls and Death Of The Family. Aside from being a neat instant visual shorthand to denote young Bruce, the new hair cut revealed a few new layers to the character. He's much more impulsive in Zero Year, prone to anger and sometimes showing a careless disregard towards people and even his work as Batman. He even gets in a war of words with Alfred at one point, asking him pointedly "What good have you done in the world?" after a callous crack about "living in a mausoleum, polishing silver". He gets a slap in the face for his troubles. To be fair, he did react angrily in response to Alfred calling him a coward and telling him that he believed Bruce's parents would be ashamed of him for letting the Wayne family name and legacy go to waste. Regardless, an older, more experienced Batman would be less likely to get into this argument with his faithful butler. Oh, and he also flips off a member of the Red Hood Gang at one point! Which we believe is a first in 75 years of Batman comics.
