The Joker would never laugh like that with a gun in his hands. He wouldn't look crazy, he'd look in control. The tats means that he's not in control. He's more just a weird guy with a gun instead of Joker, etc etc etc
well you might take into account joker's teeth were completely bashed in probably thanks to the batman. probably as a result of horribly murdering robin - whichever version that may be.
then there's the arkham asylum admittance, and all done there - probably his brains were fried like he's about to fry harleen quinzel as in the trailer(s).
not even mentioning the rumours about joker being much older and thus perhaps having a long lifespan of stuff happening. a side note here is that in BvS, lexie had some meta-human intel regarding WW, aquaman, etc.
WW is much older then she looks.....so.....who's to say jokesy cant be?
i kinda dig the idea this joker is reaching the net end result of his craze reign. well perhaps not the end result, but simply the coarse taken is 'logical'.
perhaps joker has been around really really long and he's finally met his real adversary in batman, possibly being the only one to stop him (except for superman, which somewhat makes it cheating thanks to superpowers).
besides, the joker in this scene is not crazy, well he is obviously, the joker is quite the profile of friggin crazy,
but in this scene as obvious from other trailer footage, he's rather having a blast and fun. quite like joker when crowbarring robin.....so....there you go