My general experience with "gangs" in several countries is that they try to keep their business to themselves and if someone is killed, it's usually for debts or unresolved business, member of another gang or another family, never tourists or random OCT members. Happens in Latin America, Balkans, I don't think Cambodia would be that special in this.
Also not all cities in the country would be a dirty casino hellholes. One city? Maybe. Everywhere? It's like judging the entire US based on Gary, Indiana or Clark County, Nevada...
If you are a tourist or visitor, I'd probably worry about petty theft/bag snatching and corrupt police (which can be both bad and good). Don't behave like an idiot, don't trust︀ the locals or local banks too much and you'll be fine. There are foreigners living︁ there for years without major issues.