Some people will easily hurt others because of their frustrations and when you partner it with losing in gambling it will be more serious and others are gonna be in trouble while they have a kind of that person in their house or community. I remember when I had a friend and he was good at playing pools because that's one of the sports in our community that they make bets out of while playing. My friend was just a young guy and when he won big money from those big men, they made some evil plot against him while he was going home and they took his money from him after seriously beating him in the middle of the road and that's only one example of persons that are not supposed to be playing games like that because they cannot handle the heat when they lose their money.
Hmm maybe your friend didn't gamble in a casino. It could be some indoor gambling competition between him and the big men. If it's the later, then the person was actually gambling with hot tempered gamblers. Who can't accept defeat. Or maybe the other men are cliques of friends that rip off a winner's money. That's the risk of gambling with friends, off the casino settings. The casino is there to guarantee that players would be paid after winning. Without facing any dispute with fellow gamblers. However, the offline casino is also not conducive for gamblers because of such attacks. I'd say that gamblers who win early should leave the premises, while the day is clear. This type of attacks happens during the night hours.
At least where I live pool bets are known scams and not one should participate in them, the scam is just a bait and switch scam in which the scammer fakes to play terribly and then gets in an altercation with another customer on a bar, then in order to settle their problems they play pool and make a high bet, and that is when they reveal their superior skill and scam hundreds of dollars out of unsuspecting victims, however if you were to win then they will follow you and rob you anyway, and for what I know this is a common scam all around the world so it is better to avoid those kind of bets completely.
Such games are not fair to gamblers, due to the type of people who participate in the game. I prefer playing pool with close friends, in a well-organized place. Many gamblers prefer wagering money on the game, but it's not advisable given the instance you mentioned above. It's common almost everywhere I've seen people gamble on such game. The outcome ends in chaos or a fight. However, those who participate in the game may have been used to such type of behavior from fellow players. It gets annoying to see the person who claimed to be a terrible player at first, losing the game, despite showing his super skills. He'll start the argument about his opponent not being educated enough about the rules of the game. Their aim is mainly to ruin the game or cause a fight.
Having such an experience, a gambler is not meant to be a participant in those kinds of games, as it's not conducive for everyone. They should have their type of people, those who are ready to fight over a game. Settings like that, should be avoided at all times as a responsible gambler who wants to enjoy the game, and make friends with fellow gambler. Such people also try to misbehave in casino, but the strict security rules chase them away often time, and they'll be left with no other option than to set up or spend time in bars where they can play pool games and fight over who wins or not. Although, those who rob winners in casinos, mainly stay close to where cars are parked, then rob the person once he's done gambling for the day. Preferably for casino users, it's best to go earlier during the day, or lodge in the casino hotel.