Breja: Because he does not kill? I'm not really seeing what's so silly about that.
CarrionCrow: He's silly because he uses a variation on the frog and the scorpion, and it never changes.
People get hurt, people get killed, he saves some, loses some, gets the bad guy, bad guy comes back, it starts over.
If anything, the silly part is the villains always escaping the prison/arkham, which is sort of a meta requirement- it's either that, or we don't get to use an interesting character ever again. It has little to do with Batman as a character.
In any case, Batman's motivations for his "no killing" pollicy have in fact been explored a number of times, and are strongly intertwined with his origin. Now, one can argue whether he is "right" or "wrong", and if he is being rational, but it's definately not silly. In fact it is one of the more interesting aspects of Batman, that despite being often seen as the more brutal and using more questionable methods than other heroes, he is in fact fact far more unwavering in his moral code whatever the situation than the others, and also capable of great compassion, even towards his bitter enemies. If all that was to be removed, we'd be left with a very bland vigilante guy.
So I think we're better off silly ;)
Vitek: Is anything known about what the plot and antagonist in this movie is supposed to be?
The Joker, most likely. The best guess right now is that he is up to something really big, and Waller for whatever reason need him stopped off the record. With Waller there is always something more going on, some hidden agenda, so hell knows what the actual "plot within plot" might turn out to be.