Who's going to be afraid of a guy dressed in a bat suit? Hell, what would happen if he did at Halloween. I mean in general the guy will look goofy and silly no matter what you do.
This is why he should be naked
... yeah you heard me right - naked except for the boots, glooves, belt, mask and cape.
Think about it - you are mugging some old lady in an alleyway when suddenly a guy in a gimp mask with a hard-on comes run at you - by the time you've made sense of what you have seen, he's knocked you out.