Some kids are badly behaved because they have crap parents. Some are badly behaved for very different reasons. Unless you are intimately familiar with all of the details of a particular child's life, I don't think it's appropriate to draw any conclusions.
When I was a teenager, I did some volunteering with SEN children, and I built up a particular relationship with the family of a severely autistic boy of around 11yo. He looked "normal" but was completely non-verbal, sometimes aggressive and prone to major meltdowns in which he would try to hurt himself and others around him. His mother had ended up having a bit of a mental breakdown so his father would sometimes asked if I would accompany him and his son on a day trip - he needed 2:1 care at all times because his behaviour was quite erratic and unpredictable.
Those trips were my first real insight into disability discrimination. I could see how incredibly challenging life was for the parents of this boy, who had devoted everything to caring for him. And yet they met with so much judgement on a day to day basis because the boy did not behave as others thought he should. I was truly horrified by the amount of staring, tutting and judgemental comments that we encountered, especially if the child was having a meltdown. Clearly, people just assumed that the boy was a badly behaved brat and that his dad was a crap parent... so far from the truth on both counts.
If you don't know, don't judge. You have no idea what may be going on behind the scenes...neurodivergence and/or disability, trauma, bereavement. All of these things can impact on a child's behaviour.
I save my own judgement for people who lack empathy.