In the case of Eight Belles (what an awful name to saddle a horse with--if you'll forgive the pun), I see two strikes against us: one is our questionable habit of meddling with the evolutionary process in order to satisfy our aesthetic or utilitarian needs. I've heard that the equine "thoroughbred" is so over-bred to increase lung capacity and body size that this kind of accident is unavoidable: too much weight is carried at too great a speed on too fragile legs. The second strike, of course, is that we then choose to pit these wonderful beasts against each other in order to satisfy our addiction to competition and to the thrill of speed and danger.
This is a narrow and highly specialized example of a much, much larger truth, however: we use nature in every way, to her -- and eventually to our own -- disadvantage. It is not the animals who are despoiling this planet and its delicate ecosystems. They do not use nature in this sense: they work with it, dance with it, if you will, in order to survive and perpetuate the species. It is we humans who rape, exploit, despoil and kill -- often to no purpose other than the furtherance of our own power. We have forgotten the wisdom of our ancestors, who learned to read nature, revere it, and work within its boundaries. We have become too many, too greedy, too impatient of limitations, too neglectful of our rightful boundaries.
There is a superb article on this subject, "Faustian Economics," by Wendell Berry in the May issue of Harper's magazine, to which I am unfortunately unable to provide a link (I think because you have to be a subscriber to the online version; I am a subscriber only to the hardcopy of the magazine.) Berry's thesis is that man has made a Faustian pact with the devil to indulge his concupiscence and greed, and to transcend his natural limitations -- indeed, for the right to have his every need and wish fulfilled.
The trade-off is not only man's individual soul, however; it's also the planet he has been given to live on. The most recent, truly dreadful result of our failure to treat our planet with respect and husband its resources is manifest in Myanmar (I still prefer Burma); we cannot, of course, blame every cyclone on the phenomenon of global warming, but let's not, either, absolve ourselves of all responsibility. We fool with Mother Nature at our peril.
Thus it is that we breed our thoroughbreds to race-track requirements and we race them till they drop. Then we "euthanize" them, to show how humane we are.