What can parents do if it is their kid who is the bully? The loner? The silent one who holds things in?

Get help. You need to get your child help. Find through your school or personal physician a good counselor who has experience in dealing with this problem. It is very important that you do not delay.
Talk to your kid! The most important thing a parent can do is to communicate with the child: find out what's bothering him/her, see if it's time to talk to their physician, see a therapist, discuss with school personnel.
Control a bully. School bullies regularly get away with doing things to other children for which they would do jail time if they were adults, causing great harm. Despite the nice talk about "getting them counseling", many (not all) that i've known comes from homes with poor direction and either too little or inappropriately harsh discipline. Your pastor or some other wise lay person might be a good guide. Peace.
Insecurity. Most of the time, children who bully are insecure, and are not future sociopaths. Get the child to open up about the insecurity which usually involves damaged interpersonal relationships, or, find a reputable child psychotherapist for your child to speak with.
Child behavior. The key is early detection before any behavioral problems presents.Children with behavior problems were more likely to be male and live in households with lower income and parental education. Also children with behavior problems entered kindergarten with lower speech and language, motor, play, and school skills. The latter was found by montes et al 2012. Knowning this then we can focus on prevent.