Personally I think that Gangster-theme is not more violent than other recent LEGO themes.
Then the conflict between good and evil is a quite old concept, even in older LEGO themes, such as Pirates, Castle, Space and Town too.
Police has been present in the Town-theme since many years, and I remember that in 1991, set 6679 was released in Italy as: "Smugglers' boat".
Then weapons was introduced with the Castle-theme many years ago, so I don't see the problem for the Gangster-theme to be released.
However if this wasn't enought, LEGO could still renounce to the gangsters and simply release a new Town-theme set in the '20s-'30s.
More over I love this simple old-style of building that seems to come back directly from the '80s!
What do you think about?