Well, where I work with therapeutic riding horses, I find horse racing extremely cruel. It's awful for human beings to get enjoyment from animals when the animals are in pain, suffering, and used for things other than their original intent. How can you watch a horse race and not cry when a horse collapses and has to be euthenized? It's horrible. Think of how the horse could have been put to much better use had it not been killed in a horse race. It makes a mockery of these beautiful creatures. Do me a favor and never bet on horse races. Rich snobby men do it because they have nothing better to do with their money. They do not care about the well being of the horses, nor do they even care about the sport in general. All they care about is making more money and "having a good time." Horses should not be pushed to the extent that they're foaming at the mouth and getting hurt to the point of being put to sleep. If horses had laws against cruel and unusual punishment, well this would surely be it. These races wear these horses out, and then after they're done racing, they have major medical issues in their older days, if they even live to be that age. Is horse racing any better than dog racing. No! Both are cruel, and neither should exist. If you find horse racing or any type of animal racing fun and enjoyable, that's pretty sick. Go and get another hobby!

Like someone's already said, racing itself is not cruel. The horses actually enjoy running flat out in a herd. They are bred to run. However, the treatment of racehorses by some ruthless trainers and owners is what needs to be looked at. I believe on the whole the horses are well looked after, they need to be in order to win races and make money.
If you beat and starve a racehorse he'll slow down on the course.