Returning it is the best decision that you need to do in the first place it was not your money even if it is a BTC you can still convert it into fiat money and also you will be sued by the casino and in result you will go in jail... But the good thing here is if it came in one BTC address that has been transferred to you accidentally then in my opinion it is not your fault so I will not return it.. 

But the best decision is return it to the real owner because in the end it is stealing!!