There are varying degrees of bad parenting. Of course, on the worse end is a parent who is mentally, emotionally, sexually, or physically abusive or neglectful.

Certainly, a parent who generally ignores what their children are doing, or on the other extreme, is overly dominant concerning running everything their child is doing is a bad parent.

As said before, a lack of consistency and stability can cause parenting issues.

The worst parents I see as a teacher are those who are simply completely absent from their childrens' lives and those that are abusive in any way.