Hundreds of thousands of children get mistreated each year by an adult with alcohol problems.
Alcohol diminishes judgment and causes people to lose self-control. That can lead to violence. It can cause an already violent or abusive person to become more violent or abusive, or it can cause a generally non-violent and non-abusive person to become violent or abusive.
Personally, I have found that most people who take alcohol do not become violent. In fact, most of them become nicer and more relaxed and social. However, some people get violent when they drink. I have noticed that alcohol often has the same effects on the same person almost every time they drink. In other words, a person who gets angry, confrontational, and/or violent when they drink will usually get angry, confrontational, and/or violent every time they get drunk.
To make matters worse, violent and angry people tend to drink alcohol more than less angry people.
If you notice that someone you know has a problem with alcohol and a risk of violence, you want to do something. If you know them well enough, you may want to explain your concerns and suggest that they reduce or eliminate their alcohol intake. Beyond that, you can talk to others and try to organize an intervention. Doing this when the person first becomes confrontational while drunk may allow the person to address the issue before it becomes a major problem. For example, you can help a confrontational drunk before he becomes a physically abusive drunk.
Of course, if an alcoholic physically hurts anybody, then contact the proper authorities, especially in the case of domestic abuse.