The problem with being cruel to fat people -- whether or not they deserve it -- is that a lot of them have eating disorders. Their overeating is a defense mechanism seeking comfort in the positive chemicals our brains release when we eat so we don't all starve to death. Fat people are actually much more likely to lose weight when they don't feel shamed into it.
I used to be 230 pounds, and I'm 5'4". Definitely way, way over the obesity line. No matter how ashamed I was of my body and how badly I wanted to be thin and beautiful like my sister, all my diet and exercise attempts were doomed to failure. Everytime I gave in and pigged out, I hated myself even more, and the cycle just made all my future attempts even worse.
Then I managed to get to a state where I stopped caring what people thought of my weight. I surrounded myself with positivity, realized that there are a lot of men who like a little cushion for the pushing, that there are plenty of people who look past my weight and see that I'm intelligent and funny and generous.
You know what happened then?
When I stopped thinking about how ashamed I should be of my body, I started thinking about my health, instead. I noticed that I was always tired and sore and I haven't been able to read in bed without discomfort since sometime in the 90s. For the first time, I started to try and lose weight based on things that I wanted to change about myself. I didn't want to be thin, I wanted to be healthy.
Fast forward three years later, I'm 125 lbs post breast reduction, healthy, and happy. Every bit of weight aside from my surgery, which my doctor assured me was unloseable, I lost from healthy eating and light but regular exercise. I still get ridiculous enjoyment from junk food, but I've learned to be sure I always go for a walk a day, and I feel physically better when I save it for special ocassions instead of all the time.
I could never have made this change until I overcame the feeling of being ashamed, because my feelings of shame were what turned me to overeating. You're going to find that's true of a LOT of fat people, ESPECIALLY the super, super obese ones that so often get ridiculed the most.
You want fat people to stop being fat? Come up beside them and tell them it's okay and they're free to make whatever choice they want. Because it's only when they stop hating themselves that they'll stop sabotaging themselves.
ALSO, and let me just point this out -- the threshold for where people are considered fat is ridiculously low with a lot of people, where the theshold for where people are considered skinny is much, much too lenient. A lot of the women who get crap for being "fat" have a totally normal body weight and don't need to make any changes. That's an aside, but very true.