'A lot of bad boys grow up to be good men' (WOP).
The main reason that bad boys develop into decent adults is because of good mentorship. The first mentor is usually a father or father figure. If the father is brutal and violent then it may be his example that a boy follows. If there is no family then a gang or group might replace that family. That group might be a street gang or a football supporters group or even a regiment in the army. If the leaders of those groups encourage violence and brutality in order to belong then a boy will become a violent and brutal man. If on the other hand the leaders combine their authority with strength and decency they will create strong and decent followers.
Young men don't think they are bullies they are often attempting to be like their leaders or mentors. They usually resent official authority figures such as teachers, priests and policemen. If they are going to change they have to be guided by those they admire, people they will listen to; sportsmen, soldiers and adventurers.
'Bully' is a word that everyone thinks they understand; they also think that the word doesn't apply to them, However, when an individual recommends that a bully be beaten and brutalised by the authorities or by vigilanties that person is acting out of anger for the purposes of revenge. If a bully is defeated by his victim in a fight or even hurt in the process of winning he will be deterred and I recommend that all victims learn how to defend themselves but if a figure of authority imposes violence on a young man who can't defend himself then the morality is questionable.
Most young men are scared of violence and spent extraordinary amounts of effort attempting to demonstrate the contrary. They are scared that their peers will know they are scared. It's very much an Anglo-Saxon trait linked to manhood and honour. Our young men don't have a right of passage to manhood, they don't know who to follow or how to behave. They are taught that all violence is wrong but watch their governments go to war. They are taught to walk away from trouble but are at a loss when 'trouble' runs across the street and attacks for no reason.
I had a few good mentors in the army I also had a lot of bad ones. I had a lot of good mentors in Martial Arts. I try to be a mentor today to those who are prepared to read or listen but I have to compete with anger and thoughtlessness. If you want your bad boys to be good men, find good mentors for them or be a good mentor, set a good and decent example all the time. Lead them towards admirable, experienced men. Young men need to be strong and capable in order to be confident and kind. They need strong confident, tough and kind leaders who will guide them in the best direction.
Decency has to be taught. Decent parents don't blame the state, the police or schoolteachers they look for solutions
To those who recommend violent punishment, I think you are mistaken. Violent punishment makes young men resentful and even more violent.