Big E was only a face in NXT because he's from Tampa. Attacking people while they are down, after the match is won, is a heel move. If he does that in the rest of the country people will view him as the bad guy.
Also, if they give him a monster heel push soon, he'll just end up somewhere between Ryback and Brodus Clay. His mic skills leave a lot to be desired. He doesn't really have a character or a gimmick. He doesn't have enough ring experience to handle the workload and pace of being on a major push and he will hut himself and others when his awkwardness leads to botching in longer matches when he gets tired. It won't be obvious right away because the WWE always starts these guys by having them squash Zach Ryder and 3MB, but when he gets a major PPV and screws up the feud by giving a main event star a neck injury or a concussion, it will cause him to get de-pushed.