One question. Do you still see the site around?
It was not ethical in the sense that you were a close group and you were trusted by the dev team to help them. When you saw the hole, you were supposed to inform the management about it. Anyway, ethics does not have a fix rules book. The standard varies from individual to individual.
Well, op did refer to casinos as a shady business where the owners of the site don't care or give any damn about their customers as all they want is money and more money, and would do anything to milk their customers dry - as stated by the op.
So, judging by that statement alone, it's easy to understand that even if op had any intentions of letting the casino management know about that hole, which such a mindset he developed, which made him believe that casino is a shady business and all, his intensions of letting the casino know of that whole is already defeated completely since thats one way to grow a natural hate for something or a certain group of people.
OP believes that those who run gambling casinos are heartless towards their customers and their money, so, with that hole, it's an opportunity to pay the owners of that casino back in their own coin.