I agree with Peelingmyselfofftheceiling and Tranquiliade
There is a (well-informed) school of thought though that says the Grand National has got more dangerous by bowing to public pressure and lowering the height of the fences.
Lower fences = faster race = increased danger.
So it's not a black and white issue.
I don't think the race is intrinsically cruel but it is risky. It's the longest race in the National Hunt calendar for a start and the field is huge and as such doesn't reflect the majority of races over jumps in the UK.
Everyone involved in racing from stable boys, to trainers, to course designers, stewards etc bend over backwards to eliminate as much risk as possible through protective gear, rigorous training, heavily enforced rules and course design.
You have to remember that the majority of these horses wouldn't exist in the first place if it wasn't for the racing industry. And the horses are doing what they are bred and trained to do; they are animals with a well honed flight instinct and love running in a herd.
And anyone who rides and works with horses regularly knows that they are quite accident prone animals anyway. Their confirmation puts them at risk : huge amount of weight on a small circumference of bone when galloping and jumping. They still succumb when not being ridden in races. A good friend of mine (a highly experienced and committed horse owner) recently lost two of her much-loved (and 100% healthy) horses within the space of eleven weeks. The first died because it was kicked by her paddock companion whilst out grazing one day and it had to be put down because the impact broke the bone around a joint. The second died through injuring its foot while out on a quiet hack which led to an untreatable infection. Horse ownership is fraught with anxiety - it really is. So many horses end up being destroyed in the normal course of things because veterinary science hasn't so far come up with a consistent and humane way of mending a broken leg. (Horses need four legs in order to stand up - so in order to allow a leg to heal, you end up having to drug and suspend them in a harness for six months - a very risky procedure - with no guarantee of success.)
And btw - fwiw - I don't think the incidence of stress-induced vices such as weaving, box walking, cribbing, and windsucking are much more prevalent in race horses than in other sections of the horse population. Many of those vices come about as much through boredom as anxiety - for example, in the child's pony that's kept in a stable - and only ridden on Saturdays. Generally, racing stables offer much more healthy, active and stimulating environments than that.
I do have concerns for horses that are trained very young but you can't really accuse the National Hunt fraternity of that. Most Grand National winners are on average about eight or ten years old. I think horses have to be seven years old to compete now too.
Also, the two yards I was involved with, worked on a rota system with horses in training being given regular training breaks on pasture and were brought back to fitness slowly.
As for the wastage, yes that happens. But I also personally know of three racing stables where several "retirees" are happily living out the rest of their lives at pasture. I don't know for sure but I reckon that must be replicated throughout the country. And believe me, much more work, dedication, effort and money goes in to looking after the average race horse than your average pig, sheep or calf that is raised for food.
And yes, like in any industry, there are the rogues and the shysters who are only out for a quick profit. Fortunately they are usually unsuccessful because it takes huge skill and commitment and consistent effort to be a successful trainer. The overwhelming majority of people I know who work in horse-racing work very long hours, seven days a week, in all weathers, for very little money, with their main focus being the welfare of their charges. Honestly, working in a yard is pretty tough - it's a labour of love - you have to love horses to do it.