I'm going to play devil's advocate here; I don't actually know what my own opinion about this is, and I never gamble online so I don't even know much about it or how horrible it might be. So I'm going to ask you a bunch of questions that should hopefully help you work out exactly how you got to your opinion and either change it or hold it more firmly.

Why do you say "online casinos are designed in such a way to attract addicts"? Aren't they just designed to attract people who want to use them, which happens to include addicts?

Aren't you assuming that addicts are helpless people with absolutely no agency of their own? Given that this isn't a question of chemical dependency, aren't they morally responsible for exercising better judgement? I mean, many people gamble without any problems. Why should the casino be considered immoral because of someone else's poor choices?

Is online gambling especially heinous for some reason you haven't clearly articulated, or do you apply this thinking to other things people may become addicted to as well with a similar joy/grief tradeoff? It would apply to recreational drugs, I'm certain. Someone else already asked about social media. Do you think this way about other aspects of society?