What is the worst thing parents can do to their children ?

Physically, Sexually or Emotionally abuse and/or neglect their needs.

Other things aren't "good parenting", but kid's don't come with an owners manual. Sometimes parents learn from trial and error, even if they don't mean for something to turn out one way.

But, abuse that can fall in into the catergory of physical, emotional or sexual is just not "bad parenting", but that just makes you a bad human being.