Casinos are mostly used by criminals. Thats why casino is considered indecent place. Most of the Gamblers are criminals too and they do any illegal thing for money. Drug dealing, killing, kidnapping for money, gambling and much more. I can say that gambling is the mother of all these crimes because when you get frustrated you will do such bad things.
That makes no sense at all, for the criminals to gather in one place. The casinos would have been busted already. I think known criminals hangout elsewhere. Those that you are describing are those that aren't wanted persons. They have possibly done crimes in the past but never got caught.
I really wouldn't say that such things root from gambling. Gambling roots from something else which is greed. Everything's rooted from greed. Everyone wants everything. The ones who don't want to do something illegal will give gambling a shot. It's why everyone's so crazy about the lottery. Everyone's hoping that one day their lucky day will come.
It's a messed up world. Everyone's consumed by greed. The new luxury these days is being genuinely content and happy with little things.
I really would say this mentality about crime being associated with casinos came from the initial stage where we ended up having a lot of criminals using casinos as a hang out most of the time, and over the years, which has become something of a general notion for most people when it comes to casino. However, in this day and age, a lot of things have changed.
Now, casinos are regulated, you will even need a license before you can open one, and if you really do not want to get out of business, you would really want to be watchful of the sort of activities going on within the casino, so I would say this thing is something of the old time, except of course for places where crime rate naturally is huge and then there is actually no form of regulation at all.