Gambling portrayed negatively because of the main reason gambling can have a negative effect to the person involved. And also I think gambling can lead a person to be addicted,greedy and some will commit crimes just to get a money to feed their addiction.
Yeah you are right. Most of the gamblers are also criminals. I am talking about online gamblers but traditional gamblers can do any illegal activity to get some money and gamble with that money. There are many other reasons that people portrayed gambling as a worst thing in the world. I am also against gambling no matter online or traditional gambling.