Some people might belongs to a religion that is prohibits some certain behaviors and activities, while some religion also encourages some kind of immorality and bad activity but based on the level of education or wisdom some people in those religion  have acquired, they modifies their own believes and principles based on modern wisdom, social influence, coercive and referent influence.

Come to think of it, you said that gambling is considered to be Haram in Islamic religion but to kill and die for Allah is not considerd Haram according to the teachings of Jihad (Holy war). Going forward, in 1994 a bomb was detonated in Tel Aviv ( Israel) and the bomb blow up a bus that was fill up with different individuals, 22 innocent people died and 48 people were injured. The bomb was set up by Islamic militants.

In 1995 another bomb destroyed a seven-story building in Buenos, 26 people were killed, 120+ sustained injuries, there are more of those stories. In my country, those kind of suicide bombing  has in more than 2 states are (few years ago) and killed more than 100 innocent people. All of those incidents were planned and carried out by  those Islamic people.

So, comparing gambling and killing of innocent people which is worse,  meanwhile
Jihad a sacred duties that most Muslims practice.

Humanity was not created by man, therefore one man does not have the right to take another persons life, especially if the person you are killing is innocent and has not caused any harm to anyone.

Based on what I have said on the first paragraph, some of these religion followers have been influenced with different factors and has modified their believe in a way that it suits them and doesn't cause harm to anybody. It's their life and nobody has to right to question them.

I saw that the quran talked against gambling but what was the reason why it is considered a Haram? The idea that lead to the prohibition is because it said that gambling was going to be a source of harm for another follower of the religion, family or the society and that it can not make people rich.

While it's true that some people have become addicted in gambling and they have committed some crimes just to gamble, there are also many people are still gambling responsibly and doesn't cause harm to the society, their family or anyone. They are doing perfectly okay. While some people too involved in gambling, won a life changing amount and they used the money to become financially better people. 

Another thing I would say is that people have choices in life too irrespective of their religion. Some people were born into their religion because their parents belonged to the religion but at last, they grow and gain more knowledge about some things in life and they decide to modify their beliefs.

@OP, you saw that so many people are promoting casinos on their signature but that doesn't mean that everyone of them are Muslims (Islamists). Even if they are all Muslims, who are you to question their choices? There choices in life does not harm you in any way and you are not by chance the Allah they worship.