It's often the case that children who bully other children are themselves subject to bullying behavior by their parents or siblings. This creates a vicious circle of violence that takes on a terrible momentum all of its own.
This is the unfortunate situation that Janice Avery finds herself in. On the face of it, she's a really unpleasant character, an incorrigible bully who's forever picking on other kids at school. She particularly likes bullying the smaller kids, using her size to intimidate them and steal their Twinkies. This makes Janice seem like a total coward, a really nasty piece of work.
But the true picture turns out to be more complicated than that. It transpires that Janice herself is being bullied, subject to routine beatings by her father. Here, we see a classic example of a cycle of abuse at work. Janice's father bullies her, and in turn, she bullies the kids at school.
And when we say that Janice is being beaten by her father, it's important to realize that we're talking about serious physical abuse. As Leslie tells Jess, it's the kind of abuse for which people are sent to prison.
When we realize that Janice is not just a one-dimensional bully, but someone caught up in a cycle of abuse, we feel more sympathetic towards her. This doesn't in any way excuse her appalling behavior, but it does make it easier to understand why she acts the way she does.