I'll start by saying that I'm extremely saddened at the deaths of Synchronised and According To Pete, and I do think a reduction in field size (ie number of runners) should be considered.
However, I don't agree that the Grand National or racing in general is "cruel". Cruelty would be sending a horse out to run with the intention of it being killed or injured. Everyone involved with today's runners will have wanted, hoped and prayed that their horse came back safe and sound. There is a risk, however, as there is in any activity that involves a sentient animal.
There is a lot of anthropomorphism in this thread. It should be remembered that horses have no concept of the future, of impending death or of "what might have been". Synchronised didn't "know" or "sense" anything beforehand; he simply jinked at a white line and his jockey came off. Horses don't canter down to the first fence and think "shit, I could break my neck here.". They're not running in blind terror or panic - anyone who has seen a horse bolt will recognise the difference.
There are far, far worse fates for a horse - or any animal - than a relatively pampered but short life and a quick, relatively painless death. As a society, most of us are so divorced from and scared of death that we imagine it as the worst of all possible fates.
For those who are feeling guilty about having had a bet - if you placed your bet in a betting shop, then a percentage of the bookmaker's profits go to the Levy Board, who direct hundreds of thousands of pounds every year into equine health & welfare research and into saving rare breeds on the critical list. So your bet will, in a small way, have had a positive impact on the lives of hundreds of horses and ponies.
We are a very urban society nowadays and the concept of working animals is becoming increasingly alien. Racehorses are not pets, they have a job to do and in the main enjoy that job. That doesn't preclude those involved in their care and training from forming bonds of affection and attachment to them. These people are not monsters, however convenient it might be to pigeonhole them as such. (Yes, there are a few bad apples just as there are in any walk of life, and when the authorities have sufficient evidence to deal with them, they are banned from continuing in the sport.)
There are welfare issues in racing - there are still improvements to be made in the areas of over-production and life after racing for those not successful enough or not equipped to go to stud - but what happens on the track is NOT one of those issues.
Finally - years ago I worked with a girl active in one of the more militant animal rights organisations. After the 2001 GN (run in a bog; only four finishers as most pulled up) I naively said "you must be pleased no horses were killed, anyway?". I will never, ever, forget her response. "Oh no," she said, "we like it when a horse dies, it's more ammunition and we'll get it banned quicker." Every true racing fan, every person I know who works in racing is genuinely saddened when a horse dies on the racecourse. She - a supposed animal lover - is the only person I've ever met who actually took satisfaction from it. That, to me, is more sickening than anything that could ever happen in the race.