Has a writer or artist ever been killed for their portrayal of organized crime?

It's doubtful that your work would reach an audience wide enough to give any real and established gangsters any cause for concern. Proper gangsters don't make it a habit to go after every journo or writer who might write about them, the last thing people like that want is more attention placed on them, but chances are your words would have a lot less impact than any retaliatory actions they might take. At the end of the day, they still have the same rights as anyone else and if they feel you've violated their privacy or overstepped boundaries, they'd more than likely look to pursue things in court rather than on the street. Of course, there's also the issue of how you might acquire the first-hand information and knowledge about gangsterdom to begin with. Did you think these might be the type of people to have you over for tea sometime? Allow me to tell you from experience that never crossing paths with anyone who lives that life is a far better option than its opposite. Not worth it.

Not for fictional portrayals, but journalists like Veronica Guerin and Anna Politkovskaya have been murdered for asking too many questions about various types of organised crime. Roberto Saviano has also been threatened for his works about the Camorra.

Fictional writers, not to my knowledge.

Real-life criminal organisations like the mafia, the mexican drug cartels, and so forth, generally show up in fiction all the time. This rarely provokes any sort of response.

Sometimes, though, it's a good idea to switch the names to fake ones that are obvious stand-ins for real ones, and definitely don't mention real people, as that can provoke retaliation, either as a lawsuit or more real physical revenge. For instance, some gangs do have a public presence as "legitimate" organisations- biker gangs often masquerade as legit organisations who may potentially sue (or worse). Likewise, real living people can sue for defamation (or worse).

And finally, most publishers probably wouldn't carry something that would put them at risk either.

If you're going to reflect real-life gang activity in your fiction (which is a valid thing to do), it's often a good idea to maintain at least some level of "plausible deniability"- for instance, changing the names of real people or groups to fake ones as stand-ins is often a good idea.

Yes; Veronica Guerin is an example. Journalists get killed for doing their jobs everywhere from Russia to the Philippines.

No

You now what the difference is between you and Henry Hill?

Henry Hill was an actual mobster, so he knew what he was writing about.

He wasn't in danger for writing about "gangs," he was in trouble for testifying against actual mobsters, long before he ever wrote a book.

Well Alphonse Capone ordered the death of a few journalists for snooping too much in The Outfit and write about it

Not that anyone has ever been able to prove.