I was one of those people. I don't know why, as I had a daughter of my own when I said awful things.
My ex's brother has a girlfriend with developmental problems. They had a daughter together, who also has similar problems. They are all very overweight. I used to make comments about their parenting to my ex and I hated being around the young girl because every time she asked for food it'd give me the rage. I wasn't mean to her directly, but she definitely sensed my disposition towards her.
They had another child together and she also has developmental problems. I made comments about how stupid and selfish they were to have had another kid when their first was always sent off to her grandparents to be looked after.
One day I was making nasty remarks to my ex about the first girl's weight and how I hoped our own daughter didn't get fat and my ex finally snapped and pointed out I was talking about his niece, a much loved member of the family and a child and I should shut my own fat face.
I was mortified. It did make me look at myself a bit though. I realised just how goddamn cruel I'd been. How judgmental and self righteous and flipping stupid.
Ever since that day I have managed to see past everything I thought was a negative and just see the girl herself. I never look down my nose any more, not even when she asks for food or something that used to wind me up. I don't really know what's happened to make that change but it came easily, somehow. I love her and her little sister and I stopped comparing them to my own daughter, who is herself overweight and it's all my fault for being a soft touch.
I'm utterly ashamed at myself for being THAT person and I did thank my ex for waking me up.