The Batman should not be a symbol of hope. He should be what Bruce Wayne set out to be in the comics "a creature of the night, dark, terrible". We should remember that the Batman was inspired (both in reality and fiction) by The Shadow, another dark and murderous avenger. He's about taking down the bad guys, one at a time, because he's only one, ordinary, man, unlike his sometime ally, Superman. That's the contrast. Batman wants to set the world to rights, but lacks the power: Superman has the power, but declines to use it.

As to the Riddler, it seems that his narrative arc in this movie is a familiar one. The idealist who starts by trying to remove the obviously corrupt elements of the system, before coming to believe the corruption is so systemic that the only way to cleanse it is to destroy the whole thing.

It sounds to me as if this film started out along one path, but then was forced to change direction in order to have an 'acceptable' ending.

Perhaps they should have watched V for Vendetta first.