My oldest son was diagnosed with bipolar disorder at age 5. To watch the way he acts when his meds are off or he's having mood swings, you would think that his mother and I are the worst parents in the world and he's the biggest brat. Now, he's doing well in school and acting like the great kid he is, but it's taken an incredible amount of time and work to get him there.
I've gotten all the dirty looks and rude cracks from people who have no idea what we're dealing with. Walk a mile in my shoes before you start condemning people for how their kids act. Parenting may be at fault in a lot of cases, but not all of them.