Yes it is sad, I have seen it happen in my own family. My cousin’s children were raised with every advantage and yet never could stay out of trouble. They eventually straightened out but not before causing their mother and father a great deal of heartache.
I think that when good parents do their best and the kids still have problems, you just have to assume that things might have been a lot worse without the good parenting.
And no matter how wayward a child might be in youth, that early training sticks and comes back to them later.