Why is Grandpa Portman considered a hero to the island?

Abe was hunting down and killing wights and hollowgast. But it's also very possible that having been controlled as much as he was by Miss Peregrine, and having escaped the control of the Nazis, Abe wanted to give his family as much freedom as possible. In any case, Abe Portman was a warrior, an aspect of his identity long kept secret but which, when eventually discovered, at first intimidates but then inspires his grandson.