It's not a simple black or white issue really.
The actual act of running a horse isn't cruel - it's the most natural thing for a horse. A horse is a prey animal, and relies on running to stay alive. Foals will run and run, just for fun - actually, even adults will run just for the fun of it sometimes. Some horses truly love to run, and I have no doubt that some love to race.
The greyer areas are the things like the fact that while most of England doesn't sit on a horse until it is 3 at the earliest, and many now don't until at least 4 years old, in the horse racing world, you have horses that are classed as yearlings under saddle in flat racing (jump racing is older).
Then there are the ways that these horses are kept. On the one hand, most are given treatment fit for royalty, money no object - on the other hand, a huge majority of racehorses leave racing with stomach ulcers or other vices. Then there are those who never make it out of racing, having been injured & shot. Those in racing don't always get much in the way of turnout, they live a large proportion of their lives in stables.
Then there is what happens to them after racing. While some stables do try to find good homes for them, there are a huge number of horses that come out of racing with no fixed future. And a racehorse needs retraining - it cannot just come off the track and be a normal riding horse in most cases.
Horses are big strong animals, but they are also often happy to be told what to do. So while it's true that if a horse truly doesn't want to do it, it probably wouldn't, most of them are so good natured (and possibly not that bright) that they can be pushed, persuaded or punished into doing things.
Horses on racecourses do fall and become injured, and that is a terrible thing, and not something I'd like to witness/be responsible for. But the end comes very fast - there are horse ambulances on standby, and the end will be very quick if they cannot be treated. Horses break legs running in fields by themselves, or falling over in weather like we're currently having, or being kicked by another horse - in that case, the vet may take up to an hour to get there and put them out of their misery. Which is worse? On the other hand, a horse I knew got kicked by another, leg very clearly broken beyond repair - yet he wasn't at all stressed, and stood and ate mints calmly until someone got to him and put him down - I think the adrenaline kicks in.
So, I don't know. I'm not sure what to say on racing overall. I think probably I don't think it should be banned. I'd like to see more safeguard put into place - horses not ridden so young, guaranteed retirement/rehab places for them to go to after their careers, more emphasis on a more natural way of life to reduce stress (and hence ulcer & vices). Racing has hugely benefited the horse world on the whole - the amount of money that has gone into veterinary & feeding & all sorts of research - now meaning that there have been huge breakthroughs that benefit all horses.
People shouldn't underestimate the amount of money that horse racing brings in either - it's huge. Of huge benefit to the country as a whole.