I don't think gambling can change anyone's bad manners or behavior.. Gambling is an addicting activity. doing it within a reasonable time lime is fine. but if someone gets addicted to it. I might worsen his manners even more like every other habit out there, so don't take gambling as a therapy 
Does any therapist even recommend gambling for anything? Like, because you are passing through some kind of mental challenge, they will now recommend gambling to you? I don't think that's possible, but it's possible for a therapist to tell a gambler to gamble once in a while in order to reduce his stress and boredom, which is recommendable.
But on the aspect of those who are already violent in nature and get themselves involved in any manner of ill behaviour, it's highly advisable for such a person to be taken out of gambling because the emotional stress that gambling can add to the person's current state means they can end up performing the unthinkable. The best practice for such a person is social gathering for them to socialise and adopt others way of life.