**SPOILER ALERT**
You have to remember Joker didn't count on Batman resisting the Venom and giving himself the antidote. It was supposed to be freak against freak with all of Gotham watching, which would have ruined Batman's reputation. That's my take on it, I didn't find any of the characters acting really out of character.
I was disappointed in the Killer Croc part though. All that build up to a rather simple run around. He always jumped out ahead of me so it wasn't scary, my friend said he always jumped up behind him so by the time he'd turn around Croc would almost be on top of him, leaving little time for a batarang.
That's my thoughts, but overall it is a great game. It's almost like Assassin's Creed meets Metroid Prime...with Batman. A win/win for me.