I'd probably end up dead by the end of the week from retalliation from Harley/other criminals, or at the least have the crap kicked/scared out of my by the bat family, or their allies if I did do it. Same could be said for anyone else that did it.
Can't say I'd mind some roughing up from Huntress. *Cough*
Or a Kevin Conroy voiced Batman.
Going to prison for Joker's death would be a death sentence, though. Even if they don't hate you for killing joker, they can make an easy name for themselves by killing the guy/gal that killed Joker.
And come on, like Batman wouldn't figure it out? He could probably tell you everything you did that day up to the crime. If he couldn't the league could if they could get involved. I doubt it'd be possible to get away with it. I'm not sure how many times he fails to solve such crimes.
But it's been cannon that Batman could tear down the pillars of crime. He doesn't. Know why? Balance. He keeps them infighting so they can't amass power. He's gone as far as to dictate who gets what turf in my experience. Further the harder he pushes, the harder crime pushes back. Batman seems to control crime more than get rid of it, as if anyone could erase crime forever.
With that in mind, who will fill the void Joker's death would leave? Black Mask, who's a massive psycho himself? Harley, despite all appearances, could potentially fill Joker's shoes taking control of Joker's gang.
And what sort of damage would be done in the fight for Joker's turf?
Batman's a big picture kind of guy, and I respect the hell out of that.
Joker's been assumed dead a lot in his life, but those reports turn out to be greatly exaggerated. Even if you shot Joker in the skull, and even unloaded into his skull, he'll prolly pop up later anyhow... with a bone to pick with you.
Dead, or alive, Joker could do lots of damage. Which would be the greatest amount?